Preheat the oven to 350°F and place a rack in the center.

Rinse the 18 medium-sized mushrooms and pat them dry. Use a sharp paring knife to remove and hollow out the centers of the caps; arrange the hollowed caps in a single layer in a baking dish.

Heat 1 tablespoon olive oil in a large skillet over medium heat.

Add 16 ounces lean ground chicken or turkey to the skillet. Break it up with a spatula and cook 4 to 5 minutes, until cooked through.

Add ½ medium sweet onion (finely chopped), 2 minced garlic cloves, ½ teaspoon dried thyme, ¼ teaspoon red pepper flakes, and salt and pepper to taste. Continue to cook 2–3 minutes, until the onion is softened.

Remove the skillet from the heat and stir in 4 ounces grated Gruyere cheese until combined.

Fill each mushroom cap with the chicken mixture, packing the filling into the caps and mounding it on top.

Evenly sprinkle the mushrooms with 3 tablespoons breadcrumbs total (about ½ teaspoon per mushroom).

Bake for 25 to 30 minutes, until the breadcrumbs are browned and the mushrooms are tender. Let cool briefly and serve warm.