1. Place the breadcrumbs and milk in a large bowl and stir to combine. Allow to sit for 5 minutes, then stir in the egg whites and parsley.
2. Add the ground beef, season generously with salt and pepper, then knead with your hands until fully combined.
3. Divide the meat into 36 portions, these will make the meatballs. Form each portion of meat into a disk and hold in the palm of your hand.
4. Place a cube of blue cheese in the middle of the disk, then fold the meat over the cheese and roll into a ball, so that the cheese is in the middle. Repeat with the remaining meat and blue cheese.
5. Preheat the oven to 400ºF.
6. Place a large, nonstick skillet over medium-high heat. Add the vegetable oil followed by the meatballs. Cook, turning until the meatballs are browned on 2 sides, about 4 minutes per side.
7. Add marinara sauce and water to the skillet with the meatballs and reduce the heat to low. Gently simmer the meatballs while you prepare the rest of the ingredients for the sandwiches.
8. Place the cut baguette pieces directly on the oven rack, cut side down. Toast until lightly browned, about 4 minutes. Remove and place on a cutting board. You are now ready to assemble your sandwiches.
9. Place 6 meatballs onto each of the bottom toasted bread pieces and top with a small amount of sauce. Top with parsley, onion and spring greens. Repeat with the other sandwiches.