knowledgeable foodie web users ...

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... is there a site which helps you put together a menu - eg, which would give you ideas for what main dishes go well with cauliflower cheese?

There seem to be masses that give you recipes, but I haven't found one that helps with the assembling. Are there any?

I'm not sure.
C'flower cheese is a main course in its own right in my opinion,perhaps with addition of crispy bacon bits or seeds if vegetarian.

As it's white and soft,you nedd something that contrasts in texture and colour.
Green beans?
Green salad?

As an experienced cook,one just knows what does and desn't work together.
If you give me some more ideas,I'll try to help if you like.

cauliflower cheese normally goes quite well with some roast chicken, roast tatties and something green or try with roast lamb or beef -
If you are going to serve it as a meal, then add a little green salad on the side and some lovely crusty bread.

a good way of finding out what goes with what is to google something like 'dinner menu cauliflower cheese' and it will bring up lots of hotel and restaurant menus for you to look at. having said that, i've just done it for you and like coolmama says it seems to go with everything.
i sometimes fling in some courgettes to my cauli chees and a drained can of plum tomatoes (sounds weird, but it's nice), stick some breadcrumbs over the top and serve it as a main with some green salad and crusty bread. lovely for lunch the next day if there's any left. other than that... roast beef, strips of courgette sauteed in butter, jersey royal potatoes, something like that? can you tell courgettes are in season? i'm obsessed.
good luck