In a small heavy saucepan simmer the onion, the vinegar, the water, and the ground coriander until the liquid is reduced to about 1 tablespoon and whisk in the butter, 1 piece at a time, lifting the pan from the heat occasionally to let the mixture cool adding each new piece of butter before the previous one has melted completely. (The sauce should not get hot enough to liquefy. It should be the consistency of thin hollandaise.) Season the sauce with salt and pepper, transfer it to a bowl set in a larger bowl of warm water, and keep it warm. Brush the scallops with the oil, heat a ridged grill pan over high heat until it is hot and just begins to smoke, and in it grill the scallops, for 5 minutes on each side, or until they are cooked through. Whisk the fresh coriander into the butter sauce, spoon the sauce onto 2 plates, and arrange the scallops on it.