Simmer clams in own juice a few minutes until tender. Drain and reserve liquid.
Melt butter, add flour, blend. Add clam liquid and enough milk to make 2 cups. Cook, stirring constantly, until thickened. Add potatoes, onion, and clams. Put in casserole and cover with breadcrumbs.
Place in pre-heated 350 df oven and bake for 30 minutes.