Combine garlic, lime juice, soy sauce, honey, 1 Tbsp. oil, and 1/2 tsp. sesame seeds in small bowl. Season salmon fillets with salt and place in a resealable plastic bag; add half of marinade. Seal bag, pressing out air, and let salmon sit for 30 minutes. Set aside remaining marinade.
Preheat broiler. Toss onions with remaining 2 tsp. oil on a sheet pan; broil until lightly charred, around 3 minutes. Remove salmon from marinade and place on top of green onions. Spoon some reserved marinade over salmon and broil until fish is lightly charred around edges, about 6 minutes (watch closely to avoid burning). Spoon more marinade over; garnish with chile.
Broil until salmon is charred and medium-rare at thickest pint, about 2 minutes. Garnish with watercress and finish with more sesame seeds. Serve with steamed rice and seasonal vegetables.
Beverage pairing: Sake, lager, or Sauvignon Blanc.
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