Preheat your oven to 180°C (356°F) and line a baking sheet with parchment paper.
Gently clean the mushroom caps with a damp cloth, then carefully remove the stems and chop them finely.
Heat the olive oil in a skillet over medium heat. Add the chopped stems and minced garlic, sautéing until fragrant and golden, about 5 minutes. The aroma should be earthy and garlic-y, and the mixture will look slightly reduced and glossy.
Transfer the sautéed stems and garlic to a bowl. Stir in the toasted breadcrumbs, grated Parmesan, shredded mozzarella, chopped parsley, and a squeeze of lemon juice. Season with salt and pepper to taste, mixing until well combined.
Using a spoon or piping bag, generously stuff each mushroom cap with the filling, pressing lightly to pack it in and create a rounded top.
Arrange the stuffed mushrooms on the prepared baking sheet, leaving space between each. Bake for about 20 minutes, until the tops are golden brown and bubbling, and the mushrooms are tender. You’ll notice the cheese on top melts and slightly browns, emitting a warm, savory aroma.
Remove the mushrooms from the oven and let them rest for 5 minutes. This helps the filling set and makes them easier to handle. Serve warm, garnished with extra parsley or a drizzle of olive oil if desired.