In a large skillet, melt the butter over moderate heat, add mushrooms, bell pepper, and scallions. Cook, stirring frequently, for 3 minutes. Add the sherry and simmer until reduced by half, about 5 minutes. Add the pimentos and half and half, season with salt and pepper, and bring to an almost full boil. Add the crabmeat, reduce heat to low, and stir until well blended.
Scrape into ramekins and sprinkle cheese over tops. Bake until lightly browned, about 15-20 minutes.