Heat a pan over medium heat. Add in the unsalted butter. When the butter has melted, add in the red onion and stir. Cook the onions until they turn translucent then reduce the heat to medium low. Continue to cook the onions for 20-25, stirring every few minutes, until they caramelize and become slightly jammy. Remove them from the heat and set aside for later.
Heat a separate pan over medium high heat. Add the slices of bacon to the pan and cook until the bacon is fully cooked and crispy. Transfer it to a plate lined with paper towels to cool and absorb any excess oil. Reserve 1½ tablespoons of bacon grease in the pan and discard the rest. When the bacon is cool enough to touch, roughly chop it into pieces.
Heat the same pan with the reserved bacon grease over medium high heat and add in the brussels sprouts. Season the brussels sprouts with salt and pepper. Saute the brussels sprouts until they are tender, about 5-8 minutes. Remove the pan from heat.
Preheat the oven to the temperature listed for your pizza dough. Place a pizza stone or a large sheet pan into the oven while it preheats.
Stretch the pizza dough out into a circle about 12-14 inches in diameter. Transfer the pizza dough onto the hot pizza stone or onto the hot baking sheet flipped upside down. Drizzle the olive oil onto the center of the pizza dough.
Sprinkle 1½ cups of the mozzarella and ½ cup of the parmesan onto the pizza, leaving a 1 inch border for the crust. Then layer the brussels sprouts, caramelized red onion, and bacon on top. Add the remaining ½ cup of mozzarella and ¼ cup of the parmesan on top. Bake the pizza in the oven for about 15 minutes until the pizza crust is golden brown and the cheese has melted.
Remove the pizza from the oven and allow it to cool for a few minutes before slicing. Enjoy!