1) The food at the restaurant are alterable/customizable.
2) The restaurant have customs/customizable/alterable menu. (Which one is correct? customs/customizable/alterable?)
Hope somebody will correct it for me if there's mistake. Thanks
"Food" is uncountable so you can't say "Food are customisable ..."
Do you mean that individual items on the menu can be changed by the customer when ordering? For example, if the menu includes a beetroot and feta salad topped with sesame seeds, the customer can ask the waiter to bring that dish but without the sesame seeds. Or perhaps a pasta sauce which contains small pieces of sausage could be altered for a vegetarian customer if that customer requests the sauce without the meat.
If the chef cannot cope with individual requests for changes to the stated menu, the server will say 'Sorry — no substitutions'.
Menu items can be adapted on request. Please ask your server/waiter/waitress.