Sloppy Joe Grilled Cheese Sandwiches

1 lb. ground beef
chopped onion
1-2 T. mustard
1/2 cup ketchup
1/2 cup brown sugar
slices of bread or Texas-toast bread
butter
cheese slices

Brown ground beef with onion in skillet until fully cooked.  Drain off any grease.  Add mustard, ketchup and brown sugar.  Simmer for 5 minutes.  Spread butter on one side of bread slices.  Place bread slices (butter side down) onto griddle or skillet.  Top with cheese, a scoop of beef mixture and top with another slice of bread, butter side up.  Cook over medium heat until browned.  Carefully flip over and cook other side until browned.