Preheat the oven to 190⁰ C.
Brush the lamb racks with the olive oil and rub the ground cumin all over, the season with salt and pepper. Place the lamb in a roasting tin and cook for 20-25 minutes for medium rare or leave in for linger until cooked to your liking. Remove from the oven and cover loosely with foil. Set aside in a warm place to rest for 15 minutes.
Meanwhile, make the pumpkin mash. Place the pumpkin and potatoes an a large pan of cold salted water over a high heat. Bring to the boil, then reduce the heat to low and simmer for 15-20 minutes, until tender. Drain and return to the pan. Put back on the heat for 1-2 minutes to dry out, then add the creme fraiche, cumin and coriander. Mash until smooth, then season to taste. Keep warm until needed.
Carve the lamb into individuals cutlets. Divide the pumpkin mash onto plates and top the lamb cutlets. add some steamed green beans to each plate to serve.