1 slice of onion and 1 slice tomato per hamburger
25 tomatoes, and no onions. Gives them bad breath.
It all depends, like everything in the world. If you are using Bermuda onions – thinking those are commonly used but mostly that size onion – and tomatoes that are about the same, and I would use a mandolin for slicing them mostly because it’s a lot easier. You’re going to get something like 6 – 1/4″ slices out of each onion and tomato. That comes to 100/6 =16.66 each. I’d buy 20 of each so you don’t run out. Get an onion and a tomato like you will use, and cut it how you will cut it, and count the slices. It largely depends on the size of the veggies and how you cut. But 20 should do it nicely. 8 slices each = 12.5.
It does NOT take 25 onions for 100 burgers! I’d say about 10 onions and 20 tomatoes and of course 2 heads of lettuce. 🙂
25 of each, since 1 onion quarter and 1 tomato quarter suffice for 1 burger, therefore, 1 whole onion and 1 whole tomato will supplement 4 burgers.