1-First step: heat the oven and cook the pasta. Start by heating your oven to 375°F. Bring a large pot of heavily salted water to a boil, then add 1 pound dried spaghetti noodles. Cook the pasta 1 minute less than the package directions for al dente. This helps the noodles hold their shape after baking, so the final dish stays hearty instead of mushy.

2-Second step: brown the sausage. While the pasta cooks, melt 2 tablespoons unsalted butter in a Dutch oven or large skillet over medium-high heat. Add 1 pound raw Italian sausage with the casings removed. Cook, breaking it into small pieces with a spoon, until it is no longer pink, about 5 to 7 minutes. If you want a lighter version, you can use leaner sausage, but the classic version gives the best flavor.

3-Third step: build the Tuscan-style sauce. Stir in 1 medium diced yellow onion, 1 cup julienne-cut sun-dried tomatoes, 3 minced garlic cloves, 1 1/2 teaspoons kosher salt, 1 teaspoon dried Italian seasoning, and 1/4 teaspoon freshly ground black pepper. Cook for about 4 minutes, or until the onion softens. This is where the Tuscan Baked Spaghetti starts smelling amazing, and the sauce gets that sweet, savory, garlicky base. Next, sprinkle 3 tablespoons all-purpose flour over the mixture and cook for 1 minute. This step helps thicken the sauce later. Whisk in 4 cups whole or 2% milk slowly so the sauce stays smooth. Bring it to a boil, then lower the heat and simmer until thickened, about 5 minutes. The sauce should coat a spoon nicely, but it does not need to be super thick yet because it will continue to set in the oven.

4-Fourth step: add the spinach and pasta. Stir in 5 ounces baby spinach and cook just until it wilts. Baby spinach cooks down fast, so a minute or two is usually enough. Drain the spaghetti, then add it directly to the sauce. Toss gently until every noodle is coated. If you are cooking for picky eaters, this is a good point to lift a portion before adding extra seasoning or garnishes.

5-Final step: bake until bubbly and golden. Transfer the pasta mixture to a 9×9-inch or 9×13-inch baking dish. Sprinkle the top with 3 ounces shredded low-moisture mozzarella cheese. Bake until the edges bubble and the cheese melts, about 18 to 20 minutes. Let the casserole cool for 5 minutes before serving, then finish with finely chopped fresh parsley if you want a fresh pop of color.