1-First step: Get the oven and pasta water ready Heat the oven to 375°F. At the same time, bring a large pot of heavily salted water to a boil. The salted water matters because it seasons the spaghetti from the inside out, which gives the finished dish better flavor. Once the water is boiling, cook 1 pound dried spaghetti for 1 minute less than the package directions for al dente. This slightly undercooked pasta will finish cooking in the oven, so it stays tender instead of mushy.

2-Second step: Brown the sausage While the pasta cooks, place a Dutch oven or high-sided skillet over medium-high heat and melt 2 tablespoons unsalted butter. Add 1 pound raw mild or spicy Italian sausage, breaking it up with a spoon as it cooks. Let it brown until it is no longer pink, which usually takes 5 to 7 minutes. Browning matters because it builds deep flavor and gives the sauce a savory base.

3-Third step: Build the flavor base Stir in 1 diced medium 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 this mixture for about 4 minutes, or until the onion softens. You want the onion to become tender but not browned too hard. The sun-dried tomatoes will perfume the sauce and help balance the richness of the sausage and milk.

4-Fourth step: Add the flour Sprinkle 3 tablespoons all-purpose flour over the sausage mixture and cook for 1 minute. Stir it well so the flour coats the meat and vegetables. This step helps remove the raw flour taste and sets up a smooth, creamy sauce once the milk goes in. If the flour clumps, keep stirring and break it up as you go.

5-Fifth step: Make the creamy sauce Slowly whisk in 4 cups whole or 2% milk. Adding it gradually helps prevent lumps, so take your time. Once the milk is fully mixed in, bring the sauce to a boil. Then reduce the heat and simmer for about 5 minutes, or until it thickens. The sauce should be creamy enough to coat a spoon, but still loose enough to mix with the pasta later.

6-Sixth step: Wilt the spinach Stir in 5 ounces baby spinach, about 5 packed cups, and keep stirring until it wilts down into the sauce. Spinach gives the dish a fresher taste and adds a nice color contrast against the creamy pasta. If you like, you can stir in the spinach in batches to help it blend more evenly.

7-Seventh step: Combine everything Drain the spaghetti and add it to the skillet or Dutch oven with the sauce. Toss gently until every strand is coated. Make sure the pasta is evenly mixed so each serving has sausage, vegetables, and sauce. If the mixture seems too thick, you can add a splash of reserved pasta water to loosen it a bit.

8-Eighth step: Bake until bubbly Transfer the pasta mixture to a baking dish. Sprinkle 3 ounces shredded low-moisture mozzarella cheese, about 3/4 cup, over the top. Bake at 375°F for 18 to 20 minutes, or until the edges are bubbling and the cheese has melted. For a little more color, you can leave it in just until the top looks lightly golden.

9-Ninth step: Rest and serve Let the baked spaghetti cool for 5 minutes before serving. That short rest helps the sauce settle so the portions hold together better on the plate. Garnish with finely chopped fresh parsley leaves if desired. The parsley adds a clean finish and makes the dish look extra inviting.