Place the milk and the bread in a large mixing bowl, and let stand until the bread has adsorbed all the milk. With two forks, break the bread into crumbs. Beat the ground beef into the crumbs until well mixed. Make a hollow in the center of the meat and break the eggs into it. Beat the eggs a little; then grate the onions into the eggs. Add salt, pepper and mustard. Beat the eggs into the beef. Shape into a round cake and place in the slow cooker. Drain the tomatoes, and place them on the meat. Cover and cook on Low for 5 to 7 hours. Before serving, uncover the pot; turn the heat to High, and bubble away some of the sauce. It should be thick, not thin.