A salsa recipe uses green pepper, onion, and tomato in the extended ratio 2:5:9. How many cups of onion are needed to make 96 cups of salsa?

Since the extended ratio is 2:5:9, the total number of parts is 2 + 5 + 9 = <<2+5+9=16>>16.

To find the number of cups of onion needed, we divide the number of parts for onion by the total number of parts and multiply by the total number of cups of salsa:
(5 / 16) * 96 = <<(5/16)*96=30>>30 cups.
So, 30 cups of onion are needed to make 96 cups of salsa. Answer: \boxed{30}.