Combine marinade ingredients in a large resealable plastic bag. Add lamb and place in refrigerator for 8 hours or overnight.
Remove lamb from marinade, pat dry, and set on tray. On gas grill, turn all burners to high, close lid, and heat until hot, about 15 minutes. Scrape grates clean and brush with oil.
Grill lamb, fat-side down, over medium-high heat for 25–35 minutes total depending on desired doneness, turning halfway through cooking. Aim for an internal temperature of about 145°F for medium-rare and 160°F for medium. Remove from grill and loosely cover with foil. Let rest about 15 minutes, then thinly slice. While resting, prepare salad.
In a large bowl, whisk together all dressing ingredients. Season with salt and pepper (if desired). Add kale, fennel, pomegranate seeds, or grapes, and toss to coat. Arrange dressed salad on platter and top with sliced lamb, green beans, gorgonzola cheese, and toasted walnuts.