Step 1 In a nonstick skillet, heat the olive oil over medium-high heat until hot. Add the mushrooms and sauté, stirring constantly, until they have given up their liquid and it evaporates, and the mushrooms have begun to caramelize, about 5 minutes. Stir in the onion, celery, garlic and parsley, reduce the heat and continue to cook until the vegetables have softened slightly, 5 to 7 minutes. Remove from heat and set aside to cool. Meanwhile, heat the oven to 375 degrees.
Step 2 In a large bowl, combine the ground pork and beef with the cooled vegetable mixture, fennel, red pepper, mushroom powder, bread crumbs, egg and salt, mixing until thoroughly combined. Set aside 2 cups of the meat mixture to use in the tomato sauce.
Step 3Form the remaining mixture into 12 meatballs, each roughly 2 1/2 inches in diameter. Place the meatballs on a parchment-lined baking sheet and bake until the meatballs are browned and cooked through, 15 to 20 minutes.