Preheat oven at 375°.
Stir together the mustard seeds, lemon juice, Dijon mustard, honey, bread crumbs, parsley, rosemary, and melted butter. Spread this mixture over the top of the rack of lamb. Place the lamb in a roasting pan and cook in the oven at 375° for 25 minutes.
While the lamb is cooking prepare the white bean ragout. Combine the cannellini beans, ½ cup of their reserved juices, 1 tablespoon rosemary, the soup concentrate Knorr cube and the lemon zest in a saucepan.
Bring the mixture to a simmer, stirring constantly. Reduce the heat to medium low and continue to simmer the ragout for 10 minutes. If the ragout becomes too thick, add 2 to 3 tablespoons of the reserved juices from the beans to restore the consistency.
Remove the lamb from the oven, cover and let it rest for 5 minutes before serving.