BOLOGNESE SAUCE
Step 1
In a large pot over medium-high heat, heat oil. Cook beef and pork, breaking up with a wooden spoon, until no longer pink, about 5 minutes. Drain excess fat.
Step 2
Add tomato paste to pot and cook over medium-high heat, stirring, until incorporated, about 1 minute. Add marinara sauce and stir to combine. Bring to a boil, then stir in 1 teaspoon salt. Reduce heat to medium-low and simmer, stirring occasionally, until flavors have melded, about 20 minutes; season with remaining 1 teaspoon salt, if needed.
BÉCHAMEL SAUCE
Step 1
In a medium pot over medium heat, heat milk until warm to the touch but not boiling.
Step 2
In a large pot over medium-high heat, melt butter. Add flour and cook, whisking frequently, until light tan in color and texture resembles wet sand, about 2 minutes. Whisking constantly, very slowly add milk until combined. Increase heat to high and continue to cook, whisking constantly, until sauce is thickened and smooth, 3 to 5 minutes.
Step 3
Pass spoon through edges of pot to collect any butter-flour mixture that wasn’t fully incorporated and mix to combine. Stir in nutmeg; season with salt and pepper.
ASSEMBLY
Step 1
Preheat oven to 350°. In a deep 13"-by-9" ceramic, glass, or metal baking dish, spread 1 1/2 cups Bolognese, followed by 3 pasta sheets and 3/4 cup béchamel. Sprinkle with about one-fifth of mozzarella and Parmesan. Repeat to make 5 layers, finishing with cheese.
Step 2
Spray a sheet of foil with cooking spray. Cover dish oiled side down.
Step 3
Bake lasagna 20 minutes, then increase oven temperature to 400°. Uncover and continue to bake until sides are bubbling and darkened and cheese starts to turn golden brown, 25 to 30 minutes.
Step 4
Turn broiler to medium. Broil lasagna, watching closely, until top is toasted, about 1 minute.
Step 5
Let cool 15 minutes. Top with parsley and serve.
