Preheat oven to 425F
In a medium pot or large pan, sauté the onions and garlic in butter for 2-3 mins until onions start to become translucent, then add the mushrooms, red pepper flakes, salt, and pepper and sauté 2-3 more mins until mushrooms are tender
Add the spinach (in batches if you can’t fit it all at once) and sauté until it cooks down (2-3 mins) then mix in the breadcrumbs
Transfer into a bowl and let cool while you slice your baguette and cover the bottom of two quarter baking sheets (~9x13”) with 3 Tbsp olive oil each
Once the veggies have cooled slightly, mix in the mozzarella and parmesan (⅓ cup of each); spread a slightly heaping mound of the veggie topping on each slice of bread and place on the baking sheet
Top with finely shredded mozzarella and parmesan and bake 15 mins until bread is golden brown and crispy - enjoy!
