Preheat the oven to 400F. Spray a baking sheet with oil.

Gently remove the stems, scoop out the gills and spray the tops of the mushrooms with oil, season with 1/8 tsp salt and fresh pepper.

Season steak with salt and pepper on both sides.

Spray a large skillet with cooking spray and heat on high, then add the steak and cook about 1 to 1.5 minutes on each side, until cooked through.

Transfer to a cutting board and slice thin, set aside.

Reduce heat to medium-low, spray with more oil and sauté onions and peppers 5 to 6 minutes, until soft.

Combine all ingredients in a medium bowl. Transfer to the mushroom caps, about 1/2 cup each.

Bake until the cheese is melted and the mushrooms are tender, about 20 minutes.