What veg will share a pot with tomato plants?

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I've planted my tomato plants, one each in big pots. It seems a bit of a waste to have all that compost for one plant, so is there anything edible I can grow in the pots too? Ideally I'd like to grow some lettuces. Would that work? Any other ideas? It'd need to be something that would thrive on a diet of tomato food.

Basil and nasturtium are good companion plants. Basil supposedly makes the toms grow bigger and sweeter and nasturtiums repel aphids and fungus. Both are edible, nasturtium flowers look great in salads and our toms will taste fab with the basil.
Failing that plant some French marigolds to keep whiteflies away.

Oops missed a 'y'. YOUR toms will taste fab.

Ooh good idea, thanks. :-)
Are there any vegetably-type things that'd work too?

You don't need to fill the pot. By the time your tomatoes are fully grown they will need all the compost. Tomatoes are very water hungry plants and the last thing you need to do is add other plants to the pot to use up the water the tomatoes need.