1. Preheat and Cook Pasta:
Preheat your oven to 375°F (190°C). Grease a 9×13-inch or similar 3-quart baking dish.
Cook the pasta in a large pot of salted water according to package directions until al dente. Drain and set aside.
2. Brown the Sausage:
While the pasta cooks, heat olive oil in a large, oven-safe skillet or Dutch oven over medium-high heat.
Add the Italian sausage, breaking it up with a spoon, and cook until well-browned. Use a slotted spoon to remove the sausage, leaving the drippings in the pan.
3. Sauté the Vegetables:
In the same skillet, add the diced onion and cook for 3-4 minutes until softened.
Add the sliced mushrooms and cook until they have released their liquid and started to brown, about 5-7 minutes.
Stir in the minced garlic and cook for one more minute until fragrant.
4. Create the Cream Sauce:
Pour in the chicken broth to deglaze the pan, scraping up any browned bits from the bottom.
Stir in the heavy cream, oregano, thyme, salt, and pepper. Bring to a gentle simmer.
Remove the skillet from the heat and stir in ¾ cup of the grated Parmesan cheese until the sauce is smooth.
5. Combine and Top:
Add the cooked pasta and browned sausage back into the skillet with the sauce. Stir until everything is evenly coated.
If your skillet is not oven-safe, transfer the mixture to the prepared baking dish.
Sprinkle the top evenly with the shredded mozzarella and the remaining ¼ cup of Parmesan cheese.
6. Bake to Perfection:
Bake, uncovered, for 20-25 minutes, or until the sauce is bubbly and the cheese is melted and golden brown.
For an extra crispy top, broil for the final 1-2 minutes, watching closely.
7. Serve:
Let the gratin rest for 5-10 minutes before serving. This allows the sauce to thicken slightly.
Garnish with fresh parsley and serve hot.
💡 Pro-Tips:
One-Pan Method: Using an oven-safe skillet from the start means you can assemble and bake the entire dish in one pan for easy cleanup.
Sausage Choice: For a spicier dish, use hot Italian sausage. For a milder flavor, sweet Italian sausage works perfectly.
Make it Lighter: You can substitute half-and-half for the heavy cream, though the sauce will be a bit less rich.
Vegetarian Version: Omit the sausage and use vegetable broth. Add a can of drained white beans or extra mushrooms for heartiness.
