Preheat the oven to 350°F and lightly grease a 9 × 13-inch baking dish.
Melt 2 tablespoons salted butter in a medium skillet or pot. Add the sliced mushrooms and allow them to cook without stirring for approximately 5 minutes, or until they develop a golden surface.
Season the mushrooms with the dried thyme and a pinch each of kosher salt and black pepper. Continue cooking for another 4–5 minutes, until fragrant and tender. Transfer the mushrooms from the pan and reserve them for later.
Using the same skillet, melt the remaining 6 tablespoons of butter. Stir in the chopped garlic, dried basil, and dried oregano, then cook for approximately 30 seconds, just until aromatic.
Whisk in the all-purpose flour and continue cooking for about 1 minute.
Gradually pour in the white wine, followed by the milk, whisking continuously. Bring the sauce to a boil and stir for approximately 1 minute.
Remove the skillet from the heat. Add 1/2 cup shredded mozzarella and 1/2 cup grated Parmesan, stirring until both cheeses have completely melted and the sauce is smooth. Set aside.
Bring a large pot of generously salted water to a boil over high heat.
Add the jumbo pasta shells and cook according to the package directions until al dente.
Drain the shells thoroughly and allow them to cool until they can be comfortably handled.
Combine the ricotta, shredded provolone, and well-drained spinach in a mixing bowl until evenly incorporated.
Spread approximately one-third of the prepared cheese sauce across the bottom of the greased baking dish.
Working with one shell at a time, place approximately 1 tablespoon of the ricotta-spinach filling inside each pasta shell. Arrange the stuffed shells in the prepared baking dish and repeat until all of the shells have been filled.
Scatter 1/2 cup shredded mozzarella evenly over the stuffed shells.
Pour the remaining cheese sauce over the pasta, then distribute 1/4 cup grated Parmesan across the surface. Finish by arranging the cooked mushroom-thyme mixture over the top.
Cover the baking dish with foil and bake for 25 minutes, or until the sauce is hot and bubbling.
Remove the foil and return the dish to the oven for another 15–20 minutes, allowing the surface to become golden and bubbly.
Serve the Christmas stuffed pasta warm. Garnish with fresh thyme, if desired.