Preheat the oven to 350°F. Line a baking sheet with parchment paper. Carefully twist off the mushroom stalks and chop them up finely.
Heat the olive oil in a skillet over medium heat. Add the onions, garlic, mushroom stalks, and parsley. Sauté for 5–8 minutes.
In a medium bowl, mix the Boursin, sautéed mushroom mix, half of the Parmesan, half of the breadcrumbs, lemon zest, salt, and pepper.
Use a small spoon to stuff the mushrooms with the mixture. Arrange them on the lined baking sheet.
Sprinkle the rest of the breadcrumbs and Parmesan over the stuffed mushrooms. Gently slide the tray into the oven to bake for 15 minutes until the tops of the mushrooms and crisp and golden.