Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone baking mat.
Clean the mushrooms and remove the stems. Place the mushroom caps on the baking sheet, drizzle with 2 tablespoons of olive oil, and sprinkle with 1/4 teaspoon each of salt and pepper. Toss to coat.
Bake the mushroom caps in the preheated oven for about 10 minutes, or until they begin to release their juices. Remove from the oven and set aside.
While the mushroom caps are baking, heat 1 tablespoon of olive oil in a skillet over medium heat. Add the chopped onion and garlic, and sauté until softened, about 5 minutes.
Finely chop the reserved mushroom stems and add them to the skillet. Cook until the mushrooms release their juices and become tender, about 5-7 minutes. Stir in the breadcrumbs, mixing well to combine. Cook for another 2-3 minutes.
Remove the skillet from heat and stir in the Parmesan cheese, parsley, salt, pepper, and optional dried herbs. Add the chicken or vegetable broth to the mixture to make it moist but not soggy.
Spoon the stuffing mixture into the cavity of each mushroom cap, pressing it down lightly to compact.
Place the stuffed mushrooms back on the baking sheet and bake for 15-20 minutes, or until the tops are golden and the mushrooms are tender.
Serve warm, optionally garnished with additional parsley or a sprinkle of Parmesan cheese.