Maybe sometime when I have a medium white onion on hand I can determine how much 1/20th of it weighs. Then that weight can be used as a conversion factor for any size pizza.
According to the nutritiondata.self.com website, a "large" onion on average weighs 150 grams (http://nutritiondata.self.com/facts/vegetables-and-vegetable-products/2501/2
). Armed with that information, I decided to find such a white onion at my local supermarket. When I got there, I could not find anything that was remotely close to 150 grams. And they were all larger than 5 inches. They were not large. They were not even jumbo. They were Brobdingnagian!! I was afraid I was going to have to ask the produce manager to help me carry one of those onions to the checkout aisle. Otherwise, I would have had to roll it there. I decided to look elsewhere.
Fortunately, I was able to find a large white onion at another nearby market. When I got it home, I weighed it. It was 147 grams. Without the inedible outer skins and other parts, the weight was 144 grams. One-twentieth of that (for Norma's use) comes to a bit over 7 grams. By my estimation, that is between about 2 and 3 teaspoons when chopped in the normal manner, and maybe a bit less when reduced to the size that s_b described. I don't know what "large" means to s_b or what amount of chopped white onion he uses by weight for his 12" x 17" pizza, but a few teaspoons for Norma's use seems to me to be on the low side, even for a small amount of sauce. Perhaps s_d can tell us what his next "large" white onion weighs. That way, we will be working from numbers that work for him and, therefore, enable those who decide to try his recipe to more accurately replicate what he has done.