Preheat the oven to 350°F. Line a baking sheet with parchment paper. Remove the mushrooms’ stalks and chop them up.
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 bowl mix the Boursin, sauteed mushroom mix, half the Parmesan, and half the bread crumbs with the lemon zest, salt, and pepper.
Use a spoon to stuff the mushrooms with the cheesy mix and arrange them on the lined baking sheet. Sprinkle the rest of the bread crumbs and Parmesan over the mushrooms. Gently slide the tray into the oven to bake for 15 minutes.