Cook the Pasta: Bring a large pot of salted water to a rolling boil. Add the gluten-free pasta and cook according to the package directions, aiming for al dente. Reserve ½ cup of the pasta water before draining. Drain the pasta and set aside.

Create the Creamy Sauce: While the pasta is cooking, prepare the sauce. In a medium saucepan over low heat, combine the oat milk, reserved pasta water, vegan butter, and vegan parmesan cheese. Season generously with freshly cracked black pepper. Stir constantly until the cheese is melted and the sauce begins to thicken slightly, about 5-7 minutes. Be careful not to let the sauce boil.

Assemble the Dish: Add the cooked pasta, sliced vegan sausage, and chopped spinach to the saucepan with the creamy sauce. Stir gently to combine, ensuring all the pasta is coated in the sauce.

Bake to Perfection: Transfer the pasta mixture to a baking dish (a 9x13 inch dish works well). Sprinkle the shredded vegan mozzarella cheese evenly over the top.

Bake: Preheat your oven to 350°F (175°C). Bake the pasta for 15-20 minutes, or until the mozzarella cheese is melted, bubbly, and lightly golden brown. The spinach should be wilted and the sauce should be nicely heated through.