Preheat the oven to 375°F.
Cook spaghetti in salted water until just under al dente. Drain and set aside.
In a Dutch oven, melt butter. Add sausage and cook until browned.
Stir in onion, garlic, sun-dried tomatoes, salt, pepper, and Italian seasoning. Cook until onion softens.
Sprinkle flour into the pot and stir for 1 minute. Gradually whisk in milk. Simmer until thickened.
Stir in baby spinach until wilted.
Mix cooked spaghetti with the sauce.
Spread into a 9×13-inch baking dish and top with mozzarella cheese.
Bake for 18–20 minutes until bubbly and golden.
Let cool 5 minutes, garnish with parsley, and serve.
