How to Make It
Light a grill. In a small skillet, toast the mustard and cumin seeds over moderately high heat until fragrant, about 1 minute. Let cool, then transfer the seeds to a mortar or spice grinder and grind to a powder. Rub the spices all over the lamb and season with salt and pepper.
Grill the lamb over moderately high heat, turning and rotating the racks occasionally, until lightly charred outside and medium-rare within, about 30 minutes total. Transfer the lamb to a carving board and let rest for 10 minutes.
Meanwhile, in a food processor, puree the arugula with the garlic, lemon juice and olive oil. Add the cheese and season the pesto with salt and pepper.
Carve the lamb racks into individual chops and transfer to plates. Serve the arugula pesto alongside.