Preparation

Make the sauce

• Heat the olive oil in a heavy-duty 4-quart saucepan over medium heat. Add the onion, garlic, oregano, and bay leaf. Cook, stirring often, until the onion is soft, 6 to 10 minutes.
• Add the tomato paste and cook, stirring constantly, until darkened, 3 to 4 minutes. Add the tomatoes and their juice and 1 tsp. salt. Bring to a boil and then simmer over low heat, stirring frequently, until the sauce has reduced by about a third, 40 to 60 minutes. Remove the bay leaf and season to taste with salt. Keep warm, covered. (The sauce can also be refrigerated for up to 5 days, or frozen for 1 month.)

Make the meatballs

• Position a rack in the center of the oven and heat the oven to 450°F. Coat the bottom and sides of an 8×8-inch baking dish with the olive oil. Set aside.
• Combine the ground meats, breadcrumbs, ricotta, eggs, parsley, oregano, fennel seed, red pepper flakes, and 2 tsp. salt in a large bowl. Mix gently but thoroughly with your hands.
• Divide the meat into 16 golf-ball-size portions using a 2-oz., 2-inch-diameter ice cream scoop or your hands; roll with wet hands to make them round. Arrange the balls snugly in the baking dish.
• Bake the meatballs until they register about 90°F on an instant-read thermometer and are firm to the touch, 10 to 12 minutes. (If you’ve made the tomato sauce ahead, heat it while the meatballs are cooking.)
• Remove the meatballs from the oven and drain excess fat, if there is any, from the pan. Ladle half of the sauce over them, return them to the oven, and continue to bake until an instant-read thermometer inserted into one reads 165°F, about 15 minutes. (The meatballs can be refrigerated for up to 3 days or frozen for 2 weeks.)

Cook the pasta

• Bring a large covered pot of well-salted water to a rolling boil.
• Add the pasta and cook, stirring occasionally, for 2 minutes less than package timing for al dente. Reserve 2 cups of the pasta water and drain.
• Return the pasta to the pot and mix with the remaining sauce. Add 1 cup pasta water to thin the sauce and cook over medium heat until the pasta is al dente, 2 to 3 minutes. Add more pasta water if necessary, and season to taste with salt.
• Transfer the spaghetti to a large heated serving bowl. Top with the meatballs and their sauce and serve.