For the Roquefort sauce, heat the fond and bring to the boil.
Take off the heat and crumble the Roquefort into the liquid.
Blend and mix with the cream.
Put in siphon canister and charge with two cartridges of nitrous oxide.
Arrange the sauce in the middle of the plate.
Spoon a big dollop of beef glace in the middle.
Arrange the potatoes and beef around the beef glace.