Heat the olive oil in a large Dutch oven over medium heat. Add the chopped onion and carrot. Sauté until softened.

Stir in the garlic and cook for a 1 minute, just until fragrant.

Add the ground beef and Italian sausage. Cook until browned and fully cooked, while breaking it up with a spoon.

Stir in the tomato paste and cook for a 1 minute to deepen the flavor.

Pour in the crushed tomatoes, tomato sauce, and diced tomatoes. Stir well to combine.

Season with sugar, salt, black pepper, oregano, basil, crushed red pepper flakes (if using), and the bay leaf.

Bring the sauce to a simmer. Reduce heat to low, cover partially, and let it simmer gently for at least 1 hour, stirring occasionally.

Remove the bay leaf. Taste and adjust seasoning if needed.

Serve over spaghetti and top with grated Parmesan.