When "pizza" and "onions" are used with no determiner ("the", "a", "my", "this", "that", etc.) they refer to the class of all pizzas and onions, not to particular one(s).
You cannot replace them with "one(s)" unless you refer to specific pizza(s) or onion(s):
"Q: Would you like a pizza? A: Yes I would like one."
"Q: Do you like those onions? A: Yes I like those (or them)."
"Q: Are those the onions you like? A: Yes, those are the ones."