Preheat oven to 400°F (200°C). Line a large baking sheet with parchment paper (or nonstick foil).
Clean mushrooms and blot dry. Gently press on stems to remove, then set aside the stems for later.
Brush mushroom caps with 1 tablespoon olive oil, then place gill-side down on baking sheet. Bake for 15 minutes or until tender.
When mushrooms are done baking, flip caps over (so they're gill-side up), then season lightly with salt and pepper.
Make Filling:
Meanwhile, trim ends from mushroom stems, then roughly chop.
Heat butter over medium heat in a small frying pan. When hot, add diced shallot and mushroom stems. Season with salt and pepper, then cook until browned (stirring frequently).
In a small bowl, stir together Boursin cheese and shallot-mushroom mixture.
In a separate bowl, mix remaining 2 tablespoons olive oil and panko bread crumbs. Season lightly with salt and pepper.
Stuff + Bake Mushroom:
Divide filling evenly between mushrooms caps. Then sprinkle with panko topping.
Bake mushrooms until they're hot and bread crumbs are lightly browned (10 to 15 minutes).
Sprinkle mushrooms with sliced chives and serve. Enjoy!
Notes
Mushrooms - I used about 4 (8 ounce / 227 gram) packs of button mushrooms for this recipe. Medium-sized mushrooms work best, because they shrink down to the perfect one-bite size when cooked. Cheese - You can replace the Boursin with similar cheese spreads, such as Rondelé or Alouette. Boursin also make a Dairy Free option, which is perfect for vegans!Filling the Mushrooms - When adding the filling to the caps, do not overfill them. It should be level with the cap. A little filling goes a long way!Make Ahead - You can prep the mushrooms, make the filling, and stuff the caps a day in advance. Add the panko topping just before baking them so it stays crisp.More Tips - Check out the info before this recipe card for helpful tips, variations, serving suggestions, and more!