Add 1 tbsp olive oil to a large skillet, cook the diced onions 3-5 minutes.
Combine the cooked onions, Italian sausage, breadcrumbs, parmesan cheese, egg yolk, parsley, oregano, basil, salt and black pepper in a large bowl.
Use your hands to roll the mixture into approximately 24 mini meatballs.
In the same skillet you cooked the diced onions, add an additional tablespoon of olive oil, bring to medium-high heat and place the meatballs in the skillet, brown on all sides, 3-5 minutes.
Add the meatballs to a slow cooker.
Marinara Sauce
In the same skillet, use a paper towel to whip up any grease left behind from the meatballs, then add the remaining 1 tbsp olive oil to the skillet over medium high heat.
Add the garlic, saute 1 minute, then add the diced tomatoes, tomato sauce, tomato paste, red wine, parsley, oregano, basil, salt and black pepper.
Mix everything together and simmer for 10 minutes.
Add the sauce to the slow cooker with the meatballs.
Cook on low for 2-3 hours.
Pasta
Approximately 20 minutes before serving the meatballs and marinara, bring a large pot of water to a boil and cook the pasta until al dente.
Add to the slow cooker and toss with the meatballs and sauce.