Heat oil in a large skillet over medium heat. Add spinach and sauté until wilted. Set aside.
Add chopped onions to skillet, season with salt and sauté for 10 minutes. Then add sugar and cook until they begin to caramelise, for about 5-10 minutes, adding a splash of water as needed to prevent them from drying and burning. Stir in balsamic vinegar and cook for 1 minute, set aside.
In a small saucepan, melt butter over medium heat. Add flour and minced garlic and cook, stirring constantly for 1 minute.
While whisking, slowly pour milk into the flour mixture. Season with salt and pepper and bring mixture to a boil, stirring constantly, then remove from heat.
Spread white sauce evenly over the pizza base, leaving the crust uncoated. Start with half of the cheese mix, add spinach, bacon and caramelised onions. Top with the remaining half of the cheese.
Place in the preheated oven and bake for 8-12 minutes until the cheese is melted and the crust is golden. Sprinkle with parmesan and enjoy.