For the meat: Combine the beef, pork, salt, pepper and garlic in a bowl. Mix well with your hands. Add the egg and mix thoroughly.
For cooking the cepelinai: Bring a large wide pot of salted water to a boil. Meanwhile, roll about 1-cupful of the potato mixture into a ball with your hands. Gently flatten it into a 1/2-inch-thick round patty. Place 1/4 cup of the meat mixture into the center of the potatoes. Cup your hands together to seal the potato into a football shape, gently forming into an oval. Repeat forming more cepelinais with the remaining potato and meat mixtures.
Gently slip the cepelinai into the boiling water, allowing the water to return to a boil before the next dumpling is added. Do not overcrowd the cepelinai in the pot. Stir carefully and occasionally. Boil until the dumplings are translucent and the meat is cooked through, 40 to 45 minutes. Remove the dumplings using a slotted spoon, drain and place on a heated platter.
For the bacon sauce: Cook the bacon and onions in a little olive oil in a large skillet over medium heat until the onions are lightly browned. Pour off some of the excess fat. For richer gravy, stir the sour cream carefully into the bacon-onion mixture. Blend well but do not boil. You can also serve the sour cream on the side. Spoon the bacon and onions over the cepelinais and serve.