Gather all your ingredients on a clean countertop for easy access.

Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.

Fill a large pot with salted water and bring it to a boil. Cook the pasta for 2 minutes less than the package suggests. Reserve 1 cup of the pasta cooking water, then drain the pasta and return it to the pot.

In a skillet over medium heat, cook the Italian sausage until browned, about 8 minutes. Transfer to paper towels to drain excess grease.

In the same skillet, sauté sliced mushrooms, onions, and minced garlic until the mushrooms release their moisture, about 8 minutes.

Pour soy sauce into the skillet and stir until the mushrooms absorb the flavor.

Melt 4 tablespoons of butter in the skillet over low heat. Whisk in flour, thyme, salt, and black pepper, cooking for about a minute.

Gradually stir in chicken broth, half-and-half, and Dijon mustard, cooking until thick and bubbly.

Whisk in shredded Italian blend cheeses and thawed spinach until well combined.

Combine the cheese sauce and cooked sausage with the pasta; if thick, adjust it with the reserved pasta water.

Transfer everything to the prepared baking dish. Melt the remaining tablespoon of butter and combine with panko breadcrumbs and 1 tablespoon parsley. Sprinkle over the pasta.

Bake for approximately 30 minutes or until the top is golden brown and bubbly.

Garnish with the remaining parsley before serving.