Preheat the Oven – Set the oven rack to the middle position. Preheat to 400ºF (200ºC). Line a rimmed baking sheet with foil. Grease with olive oil to prevent sticking.
Prepare the Mushrooms – Quickly rinse the mushroom caps and dry well. Alternatively, wipe away any dirt with a paper towel or pastry brush. Remove the stems, chop them, place them into a small bowl, and set aside.
Make the Stuffing - Heat a large sauté pan over medium heat. Add 1 tablespoon of olive oil and heat until hot but not smoking. Add minced garlic and onion, sauté until the onions are tender and translucent, about 2 to 3 minutes. Add the chopped mushroom stems, and sauté until tender, 1 to 2 minutes. Add the sausage, and break it into small pieces as it cooks, about 4 minutes. Add the chopped spinach, stir, and cook for 1 minute. Add the bread crumbs, stir, and cook for 2 minutes. Turn off the heat. Add the chopped parsley, basil, grated Parmesan, and mozzarella cheese, and stir to combine. Season with salt and pepper. Set aside.
Stuff Each Cap - Lightly coat each mushroom cap with olive oil, then season each side with salt and pepper. Stuff each mushroom with about 2 tablespoons of filling, pressing the stuffing into each mushroom cap.
Bake - Evenly spread rosemary and thyme (if using) on top of the prepared baking sheet. Transfer stuffed mushrooms to the greased baking sheet. Sprinkle each one with bread crumbs for extra crunch. Bake until tender, and the tops are golden brown and crispy, about 18 to 20 minutes.
Serve – Transfer the stuffed mushrooms to a platter and serve immediately while still hot. If desired, garnish with chopped parsley and parmesan cheese.
Notes
Smaller Mushrooms: Under 2 inches in diameter, add about 1 tablespoon of stuffing. Check for doneness after 15 minutes.
Make it Gluten-Free: Use gluten-free bread crumbs.
Make Ahead: The mushrooms can be filled and refrigerated for 3 days before baking.
Storing: Refrigerate in an airtight container for up to 3 days.
Reheating: Microwave on high setting in 15-second intervals until hot. Alternatively, place on a lightly greased foil-lined sheet pan and warm at 325ºF (165ºC) until hot, about 15 minutes.