Melt butter; add flour, stirring until smooth.
Add mustard, salt, Tabasco and bean liquid plus milk to make 1 1/2 cups; cook until thick.
Add beans and drained onions.
Place in a greased 1-quart casserole; border with buttered bread crumbs.
Bake in preheated 375'F. oven 30 minutes, or until bread crumbs brown.