Place the slice of bread and milk in a large bowl. Mash bread until smooth and add ground meat. egg, salt, garlic, and pepper. Knead the mixture with your hands until it is well combined.
Form 1 ½-inch meatballs, rolling a small portion of your hands. You should have 16 meatballs. Wet your hands with cold water before forming the meatballs to avoid the meat from sticking to your hands.
Many cooks in Mexico like to stuff the meatballs with a small piece of boiled egg or cheese. Others even add a small piece of a Serrano pepper. To do this, press the small piece of egg or cold cheese into the meatball and seal by rolling the meatball again between your hands.
Place tomatoes and Chipotle Peppers in your blender and process until you have a very smooth sauce. The amount given renders 2 ½ cups of sauce.
Heat the oil in a saucepan over medium-high heat. Cook onion until transparent, about 5 minutes. Add the garlic, stir, and keep cooking for 3 more minutes.
Stir in tomato sauce and simmer for 5 minutes. Add chicken broth or water if needed. Transfer meatballs to saucepan, cover, and cook for 10 minutes. Season with salt and pepper. Remove lid and keep simmering for 10 more minutes. If your sauce is too watery, remove the already cooked meatballs and place them in a warm oven, covered with aluminum foil, while you keep simmering the sauce until the desired thickness is achieved.