Prep the mushrooms – Wipe the portobello caps with a damp paper towel. Remove stems and scoop out the gills with a spoon. Brush the tops and insides with olive oil. Place on a baking sheet, gill side up.
Cook the filling – In a skillet over medium heat, sauté onion in olive oil for 3 minutes. Add garlic and red bell pepper, cooking for another 2 minutes. Stir in spinach until wilted. Add quinoa, cherry tomatoes, oregano, and red pepper flakes. Season with salt and pepper.
Stuff the mushrooms – Spoon the mixture into each portobello cap, pressing lightly. Top with feta and Parmesan.
Bake – Place the baking sheet in a preheated oven at 375°F (190°C). Bake for 20–25 minutes until mushrooms are tender and cheese is golden.
Serve – Let cool slightly before serving. Enjoy as a main dish or a hearty side.