Preheat the oven to 220 degrees.
Cut each lamb rack into half, place fat side up in a roasting pan. Place pecans, sage, garlic and oil in the bowl of a food processor & process until coarsley chopped. Season with salt & pepper & press mixture firmly onto the lamb.
Bake in the oven for 20-25 minutes for medium rare or until cooked till you like. Remove from oven and set aside for 5 minutes to rest.
Meanwhile cook potatoes in a pan of salted water over medium high heat for 4-5 minutes or until tender. Add the beans in the last minute of cooking. Drain. Place in a bowl with the onion & mayo & toss to combine.