YY to frozen this, that and the other :) I started off buying them but can't afford that now, so do my own. I'm quite the DIY instant food virtuoso these days. I buy what's on special offer and prepare them when I've enough energy - for use when they aren't or I haven't!
Mash: cut spuds in the food processor, using a grater/thin slice disc. Freeze. Freezing breaks down tissues so they can be microwaved in a minute or two.
Carrots & peppers: I haven't got an attachment that cuts into chunks so slice them by hand in front of the telly, then freeze.
Onions: Chop finely in the food processor, freeze. Good for sauces. You can do this with any fruit, too - you'll get a purée when heated. Bananas can be frozen whole or sliced (delicious still frozen!)
Tomatoes: You can freeze them whole for use in sauces.
Herbs: I freeze whole & fresh. You can crumble them once they're frozen. Things like thyme and rosemary, the leaves come off the stalks with a shake.
I depend on frozen spinach, peas and sweetcorn. You can add frozen peas, corn and pepper to rice just as it's finishing - makes 'pretty rice' with only one pan to lift for all your carbs & veg :)
The slow cooker is my second-best friend, after my electric blanket! It's on every day that I can get things together and, if I can make a lot of stuff, I freeze extras for bad days. If I cooked meat with bones in, I put it back on with water & herbs for soup.
Unfortunately the Atos people think the fact I do the above means I "cook from scratch every day" which makes me very angry - yet another example of how coping with your incapacities leaves you vulnerable to accusations of skiving! Mind you, anything would. I thought I'd change this to "I microwave a frozen dinner most evenings," which is true although I made it in the first place. But they'll have me for something else, no doubt 