How to Make It
Preheat a gas or charcoal grill to high (450°F to 500°F). Stir together cucumber, peanuts, onion, olive oil, cilantro, mint, honey, mustard, lime zest and juice, 1/2 teaspoon of the pepper, and 1/4 teaspoon of the salt in a bowl until combined. Set aside.
Sprinkle steak with 3/4 teaspoon of the salt and remaining 1/2 teaspoon pepper. Lay watermelon on its side, and cut 2 (1-inchthick) rounds from the fruit, cutting through rind and flesh. (Reserve remaining watermelon for another use.) Grease grill grates with canola oil. Place steak on grates; grill until a thermometer inserted in thickest portion of meat registers 135°F, 3 to 4 minutes per side. Transfer to a plate; cover with aluminum foil, and let rest 10 minutes.
Meanwhile, place watermelon rounds on oiled grates; grill until grill marks appear, 3 to 4 minutes per side. Transfer to a cutting board; remove and discard watermelon rinds. Cut each round into 4 wedges, and sprinkle with remaining 1/4 teaspoon salt.
Thinly slice steak across the grain. Serve with watermelon wedges, and top with cucumber salsa.