How to Make It
Brush the toast with softened butter and transfer to 4 plates. Arrange two slices of tomato on each slice of bread and season with salt and pepper.
Spread the flour in a shallow bowl and season generously with salt and pepper. In a large skillet, melt the 1/2 stick of butter. Dredge 4 of the crabs in the flour and add to the skillet. Cook over moderately high heat until browned on the bottom, about 3 minutes. Flip the crabs and cook until browned and crisp, about 3 minutes longer. Stack 2 crabs on each toast. Repeat with the remaining crabs.
Add the shallot to the skillet and cook over moderate heat, stirring, until softened, about 1 minute. Stir in the remaining ingredients and season with salt and pepper. Spoon the sauce over the crabs and serve right away.