Why this recipe works: Since we were preparing a slow-cooker recipe, we needed a way to infuse the sauce for our Slow-Cooker Meatballs and Marinara with the depth it usually gets from an hour-long simmer on the stove. Sautéing tomato paste, onions, and garlic before adding them to the slow cooker bloomed their flavors and gave the sauce a meaty base. Microwaving the meatballs before adding them to the slow cooker rendered just enough fat (which we discarded) to ensure the sauce wouldn’t be greasy. Thickening the meatballs with a panade—a paste of milk and bread—caused them to break apart. The bread absorbed too much liquid, becoming slimy and losing its binding power. We replaced the milk with cream, which added moisture and richness, and swapped out the bread for mozzarella cheese, which enriched the meatballs. It was important to use the proper amount of cheese in our Slow-Cooker Meatballs and Marinara so the meatballs didn’t taste more like mozzarella than meat.

Onion Mixture:

• 2 tablespoons olive oil
• 2 onions, chopped fine
• 1-6 ounce can tomato paste
• 6 garlic cloves, minced
• 1 tablespoon dried oregano
• ½ teaspoon red pepper flakes
• ¼ teaspoon salt

Marinara:

• ½ cup red wine
• 2-28 ounce cans crushed tomatoes

Meatballs:

• 4 ounces Italian sausage, casings removed
• 2 ounces mozzarella cheese, shredded (1/2 cup)
• 1 ounce parmesan cheese, grated (1/2 cup)
• 2 large eggs
• 2 garlic cloves, minced
• ¾ teaspoon salt
• 1¼ pounds 85 percent lean ground beef
• 3 tablespoons heavy cream
• 1 ounce parmesan cheese, grated (1/2 cup)
• 2 tablespoons finely chopped fresh basil
• salt

1. For the onion mixture: Heat oil in a Dutch oven over medium-high heat until shimmering. Add onions, tomato paste, garlic, oregano, pepper flakes, and salt and cook until softened and lightly browned, about 8 to 10 minutes. Transfer half of the onion mixture to a large bowl and set aside.
2. For the marinara: Add wine to the remaining onion mixture in pot and cook until slightly thickened, about 2 minutes. Stir in tomatoes, then transfer to a slow cooker.
3. For the meatballs: Add sausage, mozzarella, parmesan, eggs, garlic and salt to bowl with reserved onion mixture. Mash with a potato masher until smooth. Add beef and cream to bowl and knead with hands until well combined. Lightly shape mixture into 2-inch round meatballs (about 12 total). Microwave meatballs on large plate until fat renders and meatballs are firm, 4 to 7 minutes.
4. Nestle meatballs into slow cooker, discarding rendered fat. Cover and cook until meatballs are tender and sauce is slightly thickened, 4 to 5 hours on low. Let meatballs and sauce settle for 5 minutes, then skim fat from surface and stir in Parmesan and basil. Season with salt to taste. Serve.

Microwave the meatballs on a large plate or in a casserole dish to contain the rendering fat. This recipe makes enough to sauce 1½ pounds of pasta.

http://www.savorysimple.net/slow-cooker-meatballs-and-marinara-americas-test-kitchen/