Preheat oven to 350 F.
Line a 13"x 9" pan with parchment paper.
Remove any gills from mushroom caps along with any remaining untrimmed stem that may be attached and reserve. Brush the inside the caps with two Tbls garlic olive oil. Season with salt and pepper to taste and arrange mushroom caps stem side up in prepared pan.
Add 1/4 cup garlic olive oil to a large sauté pan over medium heat.
Brown sausage, breaking up any large pieces as it cooks. Add onion, reserved mushroom trimming (optional), and chili flakes to the pan. Sauté until onion becomes translucent.
Add baby spinach and sauté for two minutes until fully wilted.
Remove pan from heat and add one cup mozzarella cheese and 1/2 cup breadcrumbs to the sausage mixture, stirring to incorporate well until cheese begins to melt. Taste for seasoning and adjust accordingly with salt and pepper.
Spoon two Tbls (or enough to generously fill each mushroom cap) into the portobello caps. Top each cap with a pinch reserved mozzarella, followed by a pinch parmesan, and finish with a tsp reserved panko bread crumbs.
Bake in the center the oven for 25-30 minutes until the tops are golden brown, and the mushrooms are fully cooked through.