Preheat your oven to 375 F (190 C). Prepare a baking sheet with a silicone mat, foil, or cooking spray.

Prepare the mushrooms. Gently wipe the mushrooms with a slightly damp paper towel to remove any dirt. Do not rinse the mushrooms underwater because that will affect the texture! Once cleaned, carefully remove the stems of the mushrooms and chop the stems very small. Set the hollowed-out mushroom caps aside.

Heat 1/2 of the olive oil in a large skillet over medium heat. Add the finely chopped mushrooms shallot, celery, carrot, and garlic to the pan and stir to combine. Cook for 5-7 minutes, until the veggies have softened and the mushrooms have released their moisture. Add salt and pepper and taste, adding more to taste as necessary. Once cooked, remove from the heat and set aside.

In a large mixing bowl, add the crushed croutons, parsley, and half the parmesan. Stir to combine and then add the cooked veggies and remaining olive oil. Combine the mixture well and spoon it into the mushroom caps, placing them on the baking sheet as you fill them. Use the spoon to push the filling into the mushrooms and press the extra mixture on top. Top with the remaining parmesan and bake for 20-25 minutes, or until the mushrooms have darkened and the stuffing is golden brown.

Top with parsley for garnish. Serve the mushrooms warm and enjoy!