Cream together the mustard, tarragon, salt, pepper and butter.
Place the rack of lamb in a roasting pan and roast in a 400F oven for 10 minutes.
Take out of the oven pour the brandy over top, and turn the meat so the brandy coats the lamb all over.
Spread the creamed mixture over to of the lamb.
Put back in oven, lower heat to 375F and continue roasting until the internal temp reaches 155F (apprx 45 min- 1 hour depending on the size of the rack), remove from oven and place on a platter and keep warm (cover with foil).
Place the roasting pan direct heat.
Add Sherry,chicken broth to the pan drippings.
Scrap the borrom of the pan to get all the brown bits.
Bring to a boil, strain the sauce and serve separately.