That's a very good question! And unfortunately I don't know why, but we always say "till" about what to drink with the food:
vitt vin till förrätten
öl till maten
portvin till efterrätten
cognac till kaffet
And the same goes for parts of a dish, I've found. Thus we say that vi äter potatis till biffen ("we eat potatoes with the steak") when speaking about the components. But when speaking about the dish as a whole, we say it with med or och, as in vi äter biff med/och lök ("we eat steak and onion").