Step 1: Prepare the Mushrooms

1. Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
2. Clean the portobello mushrooms by gently wiping them with a damp cloth. Remove the stems and scoop out the gills with a spoon to create space for the filling.
3. Brush the mushrooms on both sides with olive oil and season with salt and black pepper. Place them gill-side up on the prepared baking sheet.

Step 2: Cook the Filling

1. Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the diced onion and cook for 2-3 minutes, or until softened.
2. Add the red bell pepper and zucchini, cooking for another 3-4 minutes until the vegetables are tender.
3. Stir in the chopped spinach, minced garlic, oregano, basil, smoked paprika, salt, and black pepper. Cook for 1-2 minutes until the spinach is wilted and the garlic is fragrant. Remove from heat and set aside.

Step 3: Stuff the Mushrooms

1. Divide the vegetable mixture evenly among the prepared mushroom caps, pressing the filling down gently to compact it.
2. Top each stuffed mushroom with a generous layer of mozzarella cheese, followed by Parmesan cheese. If using breadcrumbs, sprinkle them over the top for added crunch.

Step 4: Bake the Mushrooms

1. Bake the stuffed mushrooms in the preheated oven for 20-25 minutes, or until the cheese is melted, bubbly, and golden brown.
2. Remove from the oven and let cool for 5 minutes. Garnish with fresh parsley before serving.