Heat the oven to 375°F. Bring a large pot of heavily salted water to a boil over medium-high heat.

Add 1 pound dried spaghetti to the boiling water and cook according to package directions for 1 minute less than al dente. Meanwhile, make the sauce.

Melt 2 tablespoons unsalted butter in a Dutch oven or high-sided skillet over medium-high heat. Add 1 pound raw Italian sausage and cook, breaking it up into small pieces with a wooden spoon, until cooked through and no longer pink, 5 to 7 minutes.

Stir in 1 diced medium yellow onion, 1 cup julienne cut sun-dried tomatoes, 3 minced garlic cloves, 1 ½ teaspoons kosher salt, 1 teaspoon dried Italian seasoning, and ¼ teaspoon black pepper. Cook, stirring occasionally, until the onion is softened, about 4 minutes.

Sprinkle 3 tablespoons all-purpose flour over the sausage mixture. Cook, stirring constantly, for 1 minute to cook the floury taste out. Slowly whisk in 4 cups whole milk. Bring to a boil, whisking occasionally. Reduce the heat to maintain a simmer and cook, whisking occasionally, until the sauce is slightly thickened, about 5 minutes. Stir in 5 ounces baby spinach, a handful at a time, until just wilted.

When the spaghetti is ready, drain, then return to the pot. Add the sauce and toss until combined. Taste and season with more kosher salt if needed. Transfer to a 9x9-inch square (at least 2 inches high) or 9x13-inch baking dish and arrange into an even layer. Sprinkle evenly with 3 ounces shredded mozzarella cheese.

Bake until the edges are bubbling and the cheese is melted, 18 to 20 minutes. Let cool for 5 minutes. Garnish with fresh parsley leaves if using before cutting into pieces like a lasagna and serving.