Student or Learner
Bread and butter .......... his main diet.
I'd choose IS. Am I right?
Even if they weren't, I wouldn't choose "are" before "his main diet". If there were three or four unconnected foods listed, I think I would alter the word order.
"Onions, milk, bread and anchovies are his main diet" sounds extremely unnatural. (I might say "Onions, milk, bread and anchovies are the staple foods of his diet".)
"His main diet consists of/is made up of onions, milk, bread and anchovies" sounds much better.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.