• Arrange one rack in the top position and a second rack in the middle of the oven. 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 3 tablespoons unsalted butter in a medium pot over medium heat. Whisk in 3 tablespoons all-purpose flour and cook until it is golden brown and smells toasted, about 2 minutes.
• Slowly whisk in 3 cups whole milk and 1 cup heavy cream. Bring to a boil, whisking occasionally. Reduce the heat to maintain a simmer and cook until the sauce is slightly thickened, whisking occasionally, about 5 minutes. Remove the pot from the heat.
• Whisk in 4 ounces shredded mozzarella cheese, 3 ounces of the finely grated Parmesan cheese, 1/2 teaspoon garlic powder, and 1 teaspoon kosher salt until smooth.
• When the spaghetti is ready, drain. Add to the sauce and toss until combined. Taste and season with more kosher salt if needed. Transfer to an 8x8-, 9x9-, or 9x13-inch broiler-safe baking dish and arrange into an even layer. Sprinkle with the remaining 2 ounces shredded mozzarella and 1 ounce Parmesan cheese.
• Bake on the middle rack until the edges are bubbling and the cheese is melted, about 20 minutes. For additional browning, move the baking dish to the top rack. Turn the oven onto broil and broil until the cheese is browned, 3 to 4 minutes. Let cool for 5 minutes. Garnish with finely chopped fresh parsley leaves if using before cutting into pieces like a lasagna and serving.

Recipe Notes:

• Make ahead: Cover the unbaked, assembled spaghetti with aluminum foil and refrigerate for up to 2 days or freeze for up to 3 months. Thaw in the refrigerator overnight. Bake, still covered, at 350°F until heated through and bubbling around the edges, about 20 minutes. Uncover and bake until the cheese on top is golden brown, 15 to 20 minutes more.
• Storage: Refrigerate leftovers in an airtight container for up to 4 days. The baked spaghetti will lose some of its creaminess upon refrigeration. Add a splash of milk during reheating to loosen the sauce.