How to Make It
Rub meaty portion of lamb rack with pepper and 1 teaspoon of the salt. Stir together mayonnaise, thyme, and mustard in a small bowl. Rub all over meaty portion. Let stand about 10 minutes. Wrap bones on lamb rack in aluminum foil.
Preheat grill to medium-high (400°F to 450°F). Toss plums in canola oil. Place lamb, meaty side down, and plums, flesh sides up, on oiled grates; grill, uncovered, until grill marks appear, 2 minutes for lamb and 1 to 2 minutes per side for plums, cross-hatching where desired. Remove plums from grill. Flip lamb to face bone side down. Grill, covered, until a thermometer inserted in thickest portion of meat registers 140°F, about 5 minutes. Transfer to a cutting board; let rest about 10 minutes.
Stir together parsley, hazelnuts, zest, 1 tablespoon of the olive oil, and remaining 1/4 teaspoon salt in a small bowl. Cut lamb into lollipops following the bone line. Slice plum halves in half lengthwise. Serve lamb and plums with parsley-hazelnut gremolata; drizzle with remaining 1 tablespoon olive oil.