Creamed eggs on toast

1/2 C butter
1/2 C flour
1 tsp salt
1/2 tsp pepper
4 C milk
10-12 hard boiled eggs sliced
1/2 onion diced
dash dry mustard

melt butter over low heat, add onions and cook till tender. Gradually add flour, salt and pepper. Cook until smooth and bubbly. Add milk and bring to a boil over med heat, stirring constantly.  Boil 1 minute, fold in eggs. Serve over toast