Preheat your oven to 350 °F and grease a 9 × 13-inch baking dish.

Melt 2 tablespoons of butter in a large pot or skillet. Add the chopped cremini mushrooms and cook over medium heat for about 4–5 minutes. Then add the dried thyme, salt and pepper. Cook an additional 3–4 minutes, then remove the mushrooms from the skillet and set aside.

Using the same skillet, add the remaining butter along with the dried basil, oregano, and minced garlic. Cook until fragrant, about 30 seconds. Stir in the flour and cook for 1 minute to form a roux. Slowly whisk in the white wine (or broth) and the milk, stirring constantly. Bring this mixture to a boil and cook for about 2 minutes until it thickens. Remove from heat and stir in ½ cup of the mozzarella cheese until melted and smooth.

In a large mixing bowl, combine the ricotta cheese, the remaining 2 cups of mozzarella, and the drained spinach. Stir until evenly mixed.

Spoon about 1 cup of the cream sauce into the bottom of the prepared baking dish and spread it into an even layer.

Stuff each cooked jumbo shell with the ricotta-spinach mixture, and place the stuffed shells into the baking dish on top of the sauce layer. Sprinkle with the remaining mozzarella cheese, then pour the rest of the cream sauce evenly over the shells. Finally, sprinkle the grated parmesan cheese on top, and distribute the cooked mushrooms over everything.

Bake in the preheated oven for 30–35 minutes or until the sauce is bubbling and the top is lightly golden.

Remove from oven, let rest for a few minutes, then serve warm.