In a medium skillet, warm up 1 tablespoon olive oil. Once skillet is hot, add diced onions. Cook over medium heat until onions are translucent. During the last 30 seconds of cooking, add minced garlic. Toss to combine. Once garlic fragrant and onions are cooked, remove from heat and place in a small bowl. Let cool and set aside.
In a large bowl, combine torn sandwich bread, milk, parmesan cheese, ricotta cheese, and lightly whisked eggs. Then add minced parsley, ground beef and sausage. Add cooked onions and garlic mixture. Season with salt and black pepper. Using your hands, mash all the ingredients together.
Using a size 24 cookie/ice cream scoop, divide and portion out meatballs. Roll and pack the meatballs in the palm of your hands.
Brown the meatballs in batches over medium heat in an oiled skillet. No need to completely cook the meatballs, you just want to brown the outside. The meatballs will continue to cook in the sauce.
Place all the browned meatballs in a large pot. Pour in marinara sauce and let mixture come to a boil. Then reduce heat and let mixture simmer until the meatballs are warmed up and cooked throughout.