In a medium bowl, whisk to combine garlic, ginger, brown sugar, soy sauce, lemonade, lemon zest, olive oil, 1/2 teaspoon salt and 1/4 teaspoon ground black pepper; set aside.
Use a sharp knife to score the skin of the salmon, making 3-4 1-inch incisions through the skin. Place the salmon in a baking dish with sliced onions and pour marinade over. Cover and refrigerate for 1 hour, flipping halfway through.
Drain salmon and onions from marinade and discard liquid. Preheat a nonstick grill pan to medium heat, then place salmon, skin side down, into pan. Arrange onions and lemons in pan around the fish.
Grill for 8-10 minutes, then flip fish, onions and lemons and continue to grill for 5-8 minutes until fish is cooked through and onions are soft and lightly charred. Remove fish and place on a platter. Garnish with onions, grilled lemons, fresh lemon juice, chopped parsley, salt and pepper.