Prep the mushrooms: Carefully remove the stems from the mushrooms. Set the caps aside and chop the stems finely.
Sauté the mushroom stems: Melt the butter in a wide skillet over medium heat. Add the onion and garlic and gently cook until softened, about 2-3 minutes. Add the chopped mushroom stems and sauté until softened, about 5 minutes.
Finish the filling: Add the breadcrumbs and Italian seasoning to the mushrooms in the pan and stir well, to coat everything with butter. if it seems dry, add a little more butter. Cook for another 3-4 minutes, take off the heat and stir in the parmesan, parsley and salt and pepper (if needed). Then set aside to cool.
Stuff and bake the mushroom caps: Preheat the oven to 350°F. Evenly distribute the filling between the mushroom caps, pushing it slightly down as you fill them. Place the mushrooms on a baking sheet lined with nonstick silicone mats and bake until the top is browned, about 20-25 minutes.
Serve: Serve hot or warm, with a bit of lemon juice if you like.
Notes
Ingredient notes
Mushrooms: Pick evenly sized ones to make sure they bake in about the same amount of time. It's best to use ones that have a relatively large cap "opening".Breadcrumbs: I just use regular, plain breadcrumbs. But you can use Panko or Italian seasoned instead, if that's what you have on hand.Garlic: If you don't have any fresh, you can use 1 teaspoon dried garlic powder instead.Parmesan: You can add more parmesan to the filling if you want to go for the full-on cheese experience, but I don’t recommend sprinkling the stuffed mushrooms with it – the full-on exposure to the heat in the oven easily burns it.
Cooking tips
Chopping: Do chop the garlic, onion and mushroom stems quite finely.Butter amount: The cooked filling should be a bit mushy and moistened (sorry) everywhere. Add more butter/oil if there’s dry breadcrumbs left.If you want your stuffing to crisp up more, don’t press it all the way down into the mushrooms, but leave it more lose.To make ahead, make the filling, stuff the mushroom caps and chill them for up to a day. Allow the mushrooms to come to room temperature before baking.