Preheat the oven to 375°F (190°C).
Clean the mushrooms and remove the stems.
Heat 1 tablespoon of olive oil in a pan over medium heat. Add the chopped onion and minced garlic, and sauté until translucent.
Add the ground beef to the pan, season with salt and pepper, and cook until browned, breaking up any lumps with a spatula.
Remove the mixture from heat and stir in the grated Parmesan cheese.
Brush the mushroom caps with the remaining olive oil, and arrange them on a baking sheet.
Stuff each mushroom cap with the beef mixture.
Bake in the preheated oven for 15-20 minutes, or until the mushrooms are tender.
Garnish with chopped parsley before serving.