Step 1: Cook the Spaghetti - Bring a large pot of salted water to a rolling boil. Add the spaghetti and cook 1-2 minutes less than the package directions (it will continue cooking in the oven). Drain the pasta and toss it with 2 tbsp butter to prevent sticking. Set aside.
Step 2: Make the Meat Sauce - In a large skillet over medium heat, cook the ground beef (or sausage) until browned, breaking it apart with a spoon. Add the diced onion and garlic, cooking for another 2-3 minutes until fragrant and softened. Drain excess grease, then stir in the marinara sauce, Italian seasoning, salt, and black pepper. Let the sauce simmer for 5-7 minutes, stirring occasionally. Remove from heat and set aside.