Preheat oven to 400 degrees F. In a small bowl or ramekin, combine 1 tablespoon of the olive oil with the minced garlic. Stir and set aside.
In a medium skillet, heat the other tablespoon of olive oil over medium-high heat. Add the onions and mushrooms and saute for 3-5 minutes, stirring occasionally.
Place the flatbreads on a large baking sheet. Brush with the garlic olive oil. Top with shredded cheese, mushrooms, and onions. Season with salt and pepper, to taste. Place baking sheet in preheated oven and bake until cheese is melted, about 12 minutes.
While the flatbreads are baking, make the crispy sage leaves. Heat about 1/4 cup of olive oil in small skillet. When hot, carefully drop a couple of sage leaves in the hot oil. Cook for 5-10 seconds and remove from the pan with tongs. Place on a paper towel. Continue frying the sage leaves in batches. They will crisp up quickly, so make sure you watch them closely.
Place the crispy sage leaves on top of the flatbreads. Cut into pieces and serve warm.