Melt 1 tablespoon coconut oil in a grill pan over medium heat. Season sole fillet with 1 pinch salt; grill until opaque, 4 to 5 minutes per side. Trim off and discard skin.
Place butterhead lettuce in a bowl. Run a thin knife around each segment of grapefruit; scoop out using a spoon and add to lettuce. Flake sole into pieces over the grapefruit and lettuce. Add scallion and garlic.
Mix remaining 2 tablespoons coconut oil, sugar, lemon juice, salt, and pepper together in a small bowl. Pour over salad; toss gently to combine.