Step 1: Cook the Shells
Bring a large pot of salted water to a boil.
Cook jumbo shells according to package directions until al dente, about 9–11 minutes. Drain and set aside, making sure not to overcook.

Step 2: Sauté the Mushrooms
Heat olive oil in a skillet over medium heat.
Add chopped mushrooms and cook for 5–7 minutes, until softened and browned.
Add garlic and thyme, and sauté for another 1–2 minutes until fragrant.
Season with salt and pepper, then remove from heat and let cool slightly.

Step 3: Make the Filling
In a large bowl, combine the sautéed mushrooms, ricotta, mozzarella, Parmesan, and a pinch of salt and pepper.
Stir until everything is well-mixed and the filling is creamy.

Step 4: Make the Garlic Parmesan Sauce
In a saucepan, melt butter over medium heat.
Add minced garlic and cook for about 1 minute until fragrant.
Stir in the flour to create a roux and cook for 1-2 minutes.
Slowly pour in the milk (or cream), whisking constantly to prevent lumps.
Bring to a simmer and cook for 3-5 minutes, until the sauce thickens.
Stir in Parmesan cheese, and season with salt and pepper.

Step 5: Assemble the Shells
Preheat your oven to 375°F (190°C).
Spread a small layer of garlic Parmesan sauce on the bottom of a baking dish.
Stuff each cooked pasta shell with the mushroom ricotta mixture and place it in the dish.
Once all shells are stuffed, pour the remaining garlic Parmesan sauce over the top of the shells.

Step 6: Bake and Serve
Top the stuffed shells with shredded mozzarella.
Cover with foil and bake for 20–25 minutes.
Remove the foil and bake for an additional 5 minutes until the cheese is bubbly and golden.
Garnish with fresh parsley and serve hot.