Preheat oven to 350°F and line a baking sheet with parchment paper.
Place a nonstick pan over medium heat and pour oil.
Add sliced onion to heated pan and saute until fragrant.
Mix in sliced mushrooms and continue cooking until soft.
In a bowl, combine mozzarella, parmesan, egg, salt, garlic powder and mushroom mixture.
Spread mixture onto prepared baking sheet.
Bake in the oven for 15 minutes.
Change oven setting to broil.
Move baking sheet to the top rack for a few minutes to brown cheese

Slice bread with pizza cutter.
Top with arugula and drizzle balsamic reduction.