Hot Ham and Swiss Sandwiches

1 small onion, grated
½ lb. Butter
1 ½ Tb Poppyseeds
1 tsp. Worcestershire
3 Tb. Mustard
12 Slices Swiss cheese
2 lbs. Thinly shaved deli ham
1 Dozen sesame hamburger buns

Cook and stir the first 5 ingredients just until melted. Chill 1 hour. Spread generously on buns. Top with ham and cheese. Wrap each sandwich in foil and bake at 250 for ½ hour. They can also be frozen in foil – then baked at 250 for 1 hour.