Directions :

1. Arrange a rack in center of oven; preheat to 350°. In a medium pot over medium heat, heat 3 Tbsp. oil. Cook 1 1/2 cups onion, stirring occasionally, until beginning to soften and turn translucent, about 5 minutes. Add 2 Tbsp. chopped garlic and cook, stirring occasionally, until fragrant, 1 to 2 minutes more; season with 1/2 tsp. salt.
2. In a small bowl, using your hands, break down tomatoes into small- to medium-sized pieces. Add tomatoes and basil to pot; season with 1/2 tsp. salt. Cook, stirring frequently, until slightly reduced and thickened, 15 to 20 minutes. Remove from heat.
3. In a small skillet over medium-high heat, heat remaining 1 Tbsp. oil. Cook remaining 1/2 cup onion, stirring occasionally, until beginning to soften and turn translucent, about 5 minutes. Add remaining 2 Tbsp. garlic and cook, stirring, until fragrant, 1 to 2 minutes more. Add tomato paste and cook, stirring, until it turns brick red, about 1 minute; season with 1/4 tsp. salt. Let cool, then transfer to a large bowl.
4. To tomato paste mixture, add egg, egg yolk, bread, Pecorino Romano, and milk and stir until combined. Using a spatula, fold in beef and pork; season with remaining 1 tsp. salt. Form beef mixture into a 9" x 4 1/2" loaf on a parchment-lined baking sheet.
5. Spread 2/3 cup tomato sauce over top and sides; reserve remaining sauce for serving.
6. Bake meatloaf until it begins to firm up, about 30 minutes. Sprinkle with mozzarella. Continue to bake until cheese is melted and bubbly and an instant-read thermometer inserted into the center registers 160°, about 30 minutes more.
7. Top with more basil. Serve with reserved tomato sauce alongside.