Preheat the oven to 400°F and line a baking sheet with parchment or lightly grease it.
Prepare the mushrooms by trimming the dirty, woody ends from the stems and discarding them. Wipe the mushrooms clean, remove the stems, and arrange the caps hollow-side up on the baking sheet.
24 large button cremini or baby bella mushrooms
Pulse the trimmed stems in a food processor until coarsely chopped, taking care not to turn them into a paste.
Sauté the chopped mushroom stems, shallot, garlic, butter, and thyme in a skillet over medium heat for 5–7 minutes, stirring frequently, until the mushrooms have released their moisture and the mixture is fairly dry.
1 small shallot, 2 cloves garlic, 1 tablespoon butter, 1/2 teaspoon dried thyme
Add the cream cheese and stir until melted and incorporated.
2 tablespoons cream cheese
Cool the mushroom mixture for 5–10 minutes, until it is no longer hot.
Combine the cooled mushroom mixture with the raw sausage, panko, Parmesan, and black pepper, mixing just until evenly incorporated.
8 ounces mild or hot Italian sausage, 1/4 cup plain panko, 1/3 cup finely grated Parmesan, 1/4 teaspoon black pepper
Generously stuff the mushroom caps with the sausage mixture, mounding it slightly over the tops.
Bake for 18–22 minutes, until the mushrooms are tender, the filling is browned, and the sausage reaches 160°F in the center.
Rest the mushrooms for 5 minutes before serving.