The large quantity of chopped parsley—1 cup!—certainly gives the meatballs a toothsome herbal quality.
Place all the ingredients in a mini food processor and purée. Set aside.
Combine the ground meats, dry bread crumbs, eggs, egg yolks, onions, cheese, red pepper flakes, salt, and the puréed stock mixture in a large bowl and mix lightly with your hands just until the mixture is uniform; try not to overmix.
Form the mixture into 20 balls about the size of golf balls. Line them up on a sheet of waxed paper. Heat the marinara sauce in a roomy saucepan. Keep at a simmer.
In a large skillet over medium-high heat, heat about ½ inch of olive oil. Without crowding the skillet, brown the meatballs for about 10 minutes, turning them once. When they are browned to your liking, remove the meatballs from the oil using a slotted spoon and submerge them in the lightly bubbling marinara sauce. Simmer for 30 minutes, or just until the meatballs are cooked through and tender. Serve the meatballs with crusty garlic bread slices, or over spaghetti.