The latest missive on diet, brought to us this week from experts at University College London, that we should be eating seven, rather than five, portions of fruit and veg a day drew a variety of responses – mostly groans. But, with or without these findings, it makes sense to get more fruit and vegetables into our everyday diet. Vegetables are the key – yes, fruit is good but contains high levels of fructose, which our body doesn't differentiate from refined sugar. And when it comes to drinking the juice, moderation is the thing. Without the roughage fruit has in its basic form, it is really not beneficial.

So how do you get more veg into your diet? It's actually not as hard as you might think. Here are seven tips to up your fruit and veg intake without it seeming like a chore.

1) Discover the joy of onions

Onions are incredibly nutritious, and complimentary to so many dishes. I prepare big batches and keep them, in two forms, in my fridge. They both add zing to any number of dishes:

Pink citrus onions: Peel and finely slice two red onions, add a pinch of salt and toss well. Squeeze a whole lemon in – they will slowly cook in the acidity and go the most beautiful pink, and are then ready to eat. When they are very soft I pop them in a jar. They're great in salads, with boiled new potatoes, on top of grilled fish or chicken or any kind of spiced dish from a simple daal to a spiced beef casserole. Add any number of herbs, chilli and a little oil, and you have a salsa or dressing. These beauties keep refrigerated for up to five days.

Melted onions: Peel and finely slice four medium onions (brown or red, or shallots). Heat a tablespoon of oil in a heavy-based pan with a lid, add the onions and stir to coat. Add some salt to let out water, stir, add a tablespoon of apple juice and some thyme and bayleaf, heat through on a medium heat, then reduce to lowest heat and leave the onions to soften and break down for a good 30-45 minutes (stir occasionally to avoid sticking). They are cooked when a strand of onion squeezed between the finger and thumb disintegrates with no give at all. Keep this sweet mass of onions in a jar or tupperware in the fridge for at least a week and use in vegetable salads, gratins, as the beginning of a casserole or sauce, in an omelette or as the basis of a tart, bruschetta or dressing.

2) Replace potatoes with other veg

Another trick is to replace or supplement potatoes with different vegetables. If I'm craving roast spuds, I'll add lots of other colourful things (beetroot, for example). And for mash I'll use a variety of roots – celeriac, parsnip, carrot, squash, sweet potato, turnip are all brilliant. I try to work out what will go well with the dish I'm doing and then make a mix. The English classic, clapshot – roughly mashed carrot and swede with butter and a good grating of nutmeg, is great. The substitution trick works brilliantly in gratins and on pies.

3) Zing up your salads

It's easy to add tasty nuggets to a green salad: some grated carrot; apple; a few pumpkins seeds; a small segmented orange; a few radishes; finely sliced celery and fennel with a couple of spring onions; a few slices of avocado; a bit of cooked or grated beetroot. It doesn't have to be all vegetables. Blobs of goats' cheese, a piece of ham, nuts, seed or croutons help turn it into a real treat. Dressings are another oasis for extra bits and bobs – a crushed clove of garlic and or any number of herbs will complement what you have in the salad bowl (I often roughly chop mint straight in). In the summer when tomatoes are plentiful, make a great dressing by whizzing up an overripe tomato with some olive oil and a squeeze of lemon juice. The same goes for a cucumber. You'll soon find your groove.

4) Go to town with base veg

Pureed soups, pasta sauces and, to a certain extent, casseroles should be a hotbed for vegetables. As I've said, onions are a good foundation to many dishes but the classic mirepoix (a fancy name for the base veg of usually onion, leek, celery and carrot) that chefs use in braises and stocks can really boost the number of vegetables you have. Think about the flavours you want from your base and then go to town. On top of those four standards, fennel is a good fresh addition, as are peppers; for a more muted flavour but with more body, go for celeriac. Another clever ploy if you are a keen pesto maker is to bulk put the basil with a handful of spinach or kale.

5) Don't rule out frozen fruit and veg

The report was negative about the effects of eating frozen and canned fruit – but acknowledged the reasons were unclear. Frozen vegetables get a clean bill of health, and they can be an absolute godsend. Even the tiniest icebox can hold a bag of frozen peas – and squeeze a bag of frozen spinach in too. Both are brilliant for throwing into a casserole just before you serve it up. Peas and spinach cooked down with some lardons and a poached egg top also make a fantastic breakfast, especially with a few melted onions thrown in.

6) Remember the rainbow

Green leafy vegetables are top-drawer – they're the ones we should eat most of, hence the current craze for kale smoothies. But it doesn't have to be quite that hardcore. It's sprouting broccoli time at the moment – try it with a delicious dressing or sauce. Don't just go for green veg, though – try and make each meal a good mix of colours as each colour has a spectrum of health benefits. If you're planning to use either broccoli or cauliflower use both – cauliflower-and-broccoli cheese is a winner (again, with melted onions). Orange and red vegetables have their own range of benefits.

7) Eat fruit raw

Although fruit juice seems like an appealing part of healthy lifestyle choices it isn't. I often use a splash of apple juice to sweeten a dressing, a sauce or a marinade but that's about it. Eating fruit raw is best; a lot of the great classic salads mix fruit with vegetables: apples, celery and walnut; pear, chicory and blue cheese, beetroot and orange; melon, cucumber and parma ham – all of these would have been found on a 1970s buffet. And with good reason.