Caterer's Sloppy Joes

1 lb. ground beef
1/2 onion finely chopped
2 tsp. flour
1 cup ketchup
1 tbsp. sugar
1 tbsp. vinegar
1/2 tsp. salt
3/4 tsp. oregano
3/4 cup monterrey jack cheese small cubed
bread rolls
Brown ground beef in a heavy skillet. Add onion. Sprinkle on 
flour and stir well. Add ketchup, sugar, vinegar, salt and 
oregano. Simmer, stirring occasionally for approximately 40 
minutes. Remove from heat and stir in jack cheese. Pour over 
broiled bread rolls and serve with plenty of napkins.