Place butter in a medium saucepan over medium heat and allow it to melt. Once melted, add the grated onion, salt, oregano, and red pepper flakes. Once the onion has slightly browned after about 5 minutes, add the garlic, tomatoes, and sugar. Turn the heat down to low-medium and allow it to simmer until it's hearty, fragrant, and thick- about30 minutes. You'll have about 2 and 1/2 cups of sauce at this point. If you have more than that, keep simmering until the amount has reduced. Remove from heat and set aside until ready to be used. You may store the sauce in a tightly covered container in the refrigerator for up to 2 days if planning to make the pizza another day. You may freeze this sauce for later use.