Preheat oven to 375°F and line a baking sheet with parchment or a silicone mat.
Stem the mushrooms and place the caps in a bowl. Toss the stems into a food processor with the bell pepper, garlic, parsley, and olive oil. Pulse until finely chopped.
Sauté the onion in 1 tbsp coconut oil over medium heat until soft (about 8 mins). Add sausage and cook, breaking it up, for about 10 minutes until browned.
Stir in the mushroom mixture, flaxseed, salt, and pepper. Cook 3–4 more minutes until everything’s well combined.
Spray the mushroom caps with oil and place them round-side down on the baking sheet. Fill each one with stuffing and press down with the back of a spoon.
Top with Pecorino, if using, and bake for 20–25 minutes, until golden and sizzling.
Serve hot with a generous dollop of arrabbiata sauce on each one.