Preparation

• Slice off the ends of the grapefruit to expose the fruit. Stand the fruit on a cut end, and slice off the skin and pith, following the natural curve of the fruit from top to bottom. Over a bowl to collect the juice, cut on each side of each membrane to free the segments and reserve. Squeeze the membranes and the skin over the bowl to extract more juice.
• In a blender, combine 1/3 cup of the oil, the ginger, 2 Tbs. of the grapefruit juice, the garlic, lemon juice, 3/4 tsp. salt, and a pinch of pepper, and purée until smooth. Transfer to a small bowl and stir in the shallot. In a large bowl, toss the arugula, watercress, fennel, and basil.
• Pat the scallops dry. Heat a large cast-iron or other heavy-duty skillet over medium-high heat until very hot. Add the remaining 1 Tbs. oil and the butter. Season the scallops with salt. Working in batches if necessary to keep from crowding, cook the scallops, undisturbed, until very well browned on one side, 2 to 3 minutes. Flip and cook until just barely cooked through, 2 to 4 minutes more, depending on size. Transfer to a plate. Repeat, if necessary, wiping out the pan and adding more butter and oil as needed.
• Toss the greens with a pinch of salt and enough dressing to coat lightly. Arrange on a platter or plates with the avocado, grapefruit, and scallops. Drizzle with more dressing, and finish with a grind of black pepper.