Preheat the oven to 400°F (204°C). Line a baking sheet with parchment paper.
Wipe 1 pound baby bella mushrooms with a damp paper towel to clean them. Remove the stems from each mushroom and discard or save them for another use.
Spoon the entire 5.3-ounce package of Boursin cheese into the hollow of each mushroom cap. Allow the cheese to dome slightly above the cap and press gently as needed so each cap is filled.
In a small bowl, combine 1 cup panko bread crumbs, 2 tablespoons melted salted butter, and 1 tablespoon minced fresh parsley. Stir until the panko is evenly coated with butter and parsley.
Press the stuffed (Boursin) side of each mushroom into the panko mixture so the exposed cheese is well coated with the breadcrumb mixture.
Place the coated, stuffed mushrooms on the prepared baking sheet, stuffed side up, spaced so they are not touching.
Bake in the preheated oven for 10–15 minutes, or until the panko topping is golden brown and the cheese is warmed.
Remove from the oven and let the mushrooms cool for a few minutes before serving warm.