Pour off and discard all but a very thin layer of fat from the skillet. Take the skillet off the heat and add the Cognac. Carefully return the skillet to the heat – the alcohol should immediately burst into flames (not a bad thing!); if it doesn’t, ignite the Cognac with a long match or lighter. Once the flames have subsided, lower the heat to medium, add the shallots and a pinch of salt and cook, stirring, until the raw shallot aroma disappears, about a minute. Whisk in the mustard, crème fraîche, lemon juice and water. Season the sauce to taste with salt, and add more water if you prefer a looser consistency. Remove from the heat.