Philly Cheesesteak-Stuffed Portobello Mushrooms

Preheat oven to 350 degrees F. Place mushroom caps on a large rimmed baking sheet and brush all sides lightly with 1 tablespoon oil. Arrange the mushrooms stem-side up.

Heat 1 tablespoon oil in a large skillet over medium heat. Add onion and cook, stirring, until starting to soften, about 3 minutes. Add peppers, 1/4 teaspoon pepper and 1/8 teaspoon salt; cook, stirring, until the peppers are softened, 4 to 5 minutes. Transfer the vegetables to a plate.

Season beef with the remaining 1/4 teaspoon pepper and 1/8 teaspoon salt. Heat the remaining 1 tablespoon oil in the skillet; add the beef and cook, stirring, until browned on all sides, 2 to 3 minutes. Return the vegetables to the pan and stir to combine. Fill each of the prepared mushrooms with about 3/4 cup filling and top with 3/4 ounce cheese. Bake until the mushrooms are tender and the cheese has melted, 12 to 15 minutes. Sprinkle with parsley before serving, if desired.