Preheat the boiler.
In a small saucepan, heat the broth over high heat until it simmers. Put the flour in a small bowl or ramekin, spoon 3 tablespoons of the broth into it, and whisk it to a smooth paste with a fork. Pour the rest of the hot broth into a bowl and reserve both broth and paste.
Return the saucepan to the heat and melt the butter over medium-low heat until it's frothy and add the shallot, salt, black pepper, and nutmeg. Cook, stirring occasionally, until the shallot is fragrant and translucent, but not brown, about 3 minutes. Add the cream, sherry, the reserved broth, and reserved flour paste mixture, and whisk to combine. Bring to the gentlest simmer and cook, stirring occasionally, until the cream sends up thick bubbles and is thickened to the consistency of a gravy, 6 to 8 minutes. Add the crab meat and cook just until it is heated through and the sauce coats the crab meat thickly, about 4 minutes.
With a slotted spoon, divide the crab meat between two 6-to-8 ounce gratin or brûlée dishes and put them on a rimmed baking sheet. Spoon the crab gravy over the crab meat just shy of covering the meat (you may have some left over for sopping with bread; lucky you!). Sprinkle the cheese over the top of the casseroles, and broil about 2 inches from the heat, until the cheese is gently browned and bubbling, about 3 minutes. Remove the casseroles from the oven and dust with paprika, if using.
Serve with spoons for scooping the casserole onto the toast.