Preheat oven to 375 degrees F (190 degrees C).

Melt 2 tablespoons of butter in a large nonstick skillet over medium-high heat. Add onion and 1/4 teaspoon salt; cook until softened, about 4 minutes.

Add Italian seasoning and 1 teaspoon garlic; cook, stirring constantly, until fragrant, about 1 minute.

Add ground meat mix and 1 teaspoon salt; cook, stirring occasionally to crumble, until browned and cooked through, 6 to 8 minutes.

Add marinara sauce and basil; cook, stirring occasionally, until warmed through, about 3 minutes. Remove from heat.

Bring a large pot of lightly salted water to boil. Cook spaghetti until tender yet firm to bite, about 12 minutes. Drain.

Melt remaining 2 tablespoons butter in saucepan over medium heat. Add remaining 2 teaspoons garlic; cook, stirring constantly, until fragrant, about 1 minute.

Add heavy cream; cook, whisking often, until thickened and coats the back of spoon, about 8 to 10 minutes.

Gradually whisk in Parmesan cheese and remaining 1/2 teaspoon salt.

Transfer pasta and cream mixture to greased 13x9 baking dish; toss gently to combine.

Spread meat sauce over pasta, sprinkle mozzarella, and add crushed red pepper if desired.

Bake in preheated oven until bubbly and mozzarella is lightly browned, about 25 minutes.