Heat 1 tablespoon olive oil in a large pot over medium heat. Add the onions and sauté for 4 minutes. Add the garlic and cook for 1 more minute. Remove and let cool.
Combine the whisked eggs, half and half, and Worcestershire sauce in a large bowl. Add the crushed Ritz crackers and stir for 1-2 minutes. This is called a “panade” and will make the meatballs super flavorful and tender. Add Parmesan cheese, parsley, Italian seasoning, mustard powder, salt, pepper, and cooled onions/garlic and stir to combine.
Add the ground beef, sausage, and pork and gently work the meat until evenly combined. (Don’t overwork the meat, this can make the meatballs tough.)
Roll into 1 ½-inch meatballs. (Pro Tip: Coat your hands in a little olive oil to make rolling easier, the meat won’t stick to your hands as much.)
Refrigerate for 10-15 minutes to firm up the meatballs. Then give them each one more roll to smooth them out.
Add the flour on a large plate and roll each meatball in it to lightly coat the outside. This helps it stay together during cooking.
Heat remaining 3 tablespoons olive oil over medium heat in the same pot that you cooked the onions in. Brown the meatballs in batches, leave plenty of space around them so that you can easily roll them to brown on each side. Brown for 1 ½ minutes per side, about 5-6 minutes total. Set aside on a plate once browned.