Directions

1. Gather all ingredients.
2. Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.
3. Cook pasta in a large pot of boiling salted water 2 minutes less than the suggested time on the package directions. Drain, reserving 1 cup pasta cooking water. Return pasta to pot and keep warm.
4. Meanwhile, cook sausage in a 12-inch skillet over medium heat until browned, about 8 minutes. Transfer to paper towels to drain using a slotted spoon.
5. Add mushrooms, onion, and garlic to the skillet. Cook until liquid is released and has evaporated, and vegetables are tender, about 8 minutes.
6. Pour in soy sauce to the skillet. Cook and stir until soy sauce is absorbed by mushrooms, about 1 minute.
7. Transfer mushroom mixture to pot with the pasta.
8. Add 4 tablespoons butter to the skillet. When melted, whisk in flour, thyme, salt, and pepper. Cook and stir for 1 minute.
9. Add broth, half-and-half, and Dijon mustard to the skillet. Cook and stir until thickened and bubbly. Whisk in cheese and spinach until cheese is melted.
10. Add sauce and sausage to the pot with the pasta mixture, stir to combine. Thin sauce to desired consistency with reserved pasta water. Transfer to the prepared baking dish.
11. Melt remaining 1 tablespoon butter. Stir in panko and 1 tablespoon parsley. Sprinkle panko mixture over the pasta mixture.
12. Bake in the preheated oven until browned and bubbly, about 30 minutes.
13. Garnish with remaining 1 tablespoon parsley.