To grow properly, the union requires a lot of light and warmth, and a rich but not too dry soil, which should be in the neutral to alkaline range. The onion has been cultivated for a very long time in the Steppes of the western and middle parts of the Asiatic area. The earliest historical records of the use of onions agriculturally are from Egypt. It was the Roman culture that spread the onion throughout Europe. They have very high vitamin content.
Mushrooms can go just about anywhere on a pizza.
Then they added green peppers and black olives. Still not satisfied, they put ham on there. The green pepper is actually an unripened fruit of pepper plants of the genus, Capsicum. Some people don't like green peppers on their pizza. Those who do, however, like them to still be a little crisp, and not all mushy or covered with sauce and cheese and stuff.