Preheat oven to 400°F (200°C). Boil potatoes until tender, then drain.
Melt butter in a saucepan until golden brown. Mash potatoes with brown butter, milk, salt, and pepper.
Heat olive oil, sauté onion, add garlic, then mushrooms and bell pepper. Cook until moisture evaporates.
Add tomato paste, smoked paprika, cumin, salt, and pepper. Pour in vegetable broth and simmer until thickened.
Spread mushroom filling in a baking dish, top with mashed potatoes, create texture with a fork, then bake for 20–25 minutes. Garnish with parsley and smoked paprika before serving.
Notes
For extra flavor, sprinkle Manchego cheese on top before baking.