Melt the butter in a frying pan over medium heat.
Add the diced onions and cook, covered, stirring occasionally, until softened and golden brown (about 20-25 minutes).
Remove from heat and set aside.
Spread the pre-baked pizza crust with your desired amount of ricotta cheese.
Top with a layer of mozzarella cheese.
Arrange the caramelized onions evenly over the cheese.
Dollop the remaining mozzarella and white cheddar cheese in small portions around the pizza (instead of spreading).
Preheat your oven to the temperature recommended for your pizza crust (usually around 400-450°F).
Bake the pizza until the crust is golden brown and the cheese is melted and bubbly (around 10-15 minutes).
Remove the pizza from the oven and slice it into wedges.
Enjoy hot!