You can put almost any vegetables into a stock to create a beautiful, aromatic foundation for recipes. Onions, leeks, carrots, parsnips and celery are popular, as are the trimmings of dark green leafy vegetables. Adding tomatoes will alter the taste and appearance. Adding potatoes, mushrooms or eggplant, for example, may cause the stock to be cloudy and these vegetables can also absorb the liquid (just like a sponge) rather than infuse it.
Try not to make your stock too pungent by adding ginger, garlic or chilli as this may overpower the recipe you want to ultimately add your stock to. Ideally, it should be a light, fragrant broth. If you have leftover basil it will add a light herbaceous note, otherwise herbs such as thyme, rosemary or oregano are often used. No fresh herbs? Use a teaspoon of dried and check the flavour before you strain well.