Melt butter, add onion, and cook over moderate heat until onion is soft. Add flour, stir to blend, cook 1 minute; stir in broth, cooking until smooth. Add crabmeat, simmer 8 to 10 minutes. Remove from fire; cool.
Cheese Topping: Melt butter, add flour to make a roux, and cook two or three minutes. Add cheses, blend well. Remove from fire, cool, then form into small firm, round balls.
Cut bread into canape-sized pieces; saute in butter. Spread each with the crab mixture; then place a cheese ball over the crab. Just before serving, place in hot oven for 5 minutes until cheese is melted and bubbly.
Makes about 50.