Sauté onions and mushrooms in half the butter in a large skillet over medium-low heat, until onions have become limp but not brown.
Add crabmeat, pimiento, and sherry and turn heat up a bit. Cook while agitating pan (don't stir), until the sherry is boiled away.
Melt the remaining butter into the mixture, and pile onto rounds of toasted French bread in ovenproof dishes. Run the plates under the broiler, about five inches from the heat, until the crabmeat sizzles. Sprinkle with parsley.