Combine the milk and vinegar in a bowl and let sit for about 10 minutes until it begins to curdle. (Alternatively, you can use buttermilk.) Let the sliced onions sit in this mixture for about one hour or up to one day, refrigerated.

Heat oil in a heavy-bottomed saucepan with sides at least three inches (seven and a half centimeters) high.

Combine flour and seasonings in a shallow bowl. Remove onions from the milk and shake off the excess liquid. Toss in small batches in the flour mixture.

Being careful not to overcrowd the pan, fry the onions in hot oil for about two minutes. Remove with a slotted spoon and cool on a cooling rack.

Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius). 

Place a mini pizza crust on a work surface. Top with a cooled onion ring. Arrange sauce, cheese, and topping on top. 

Bake for approximately 10 minutes, or until cheese starts to bubble and turns golden brown.