Scoring the scallops before grilling makes them open up like a flower, and the contrast between the golden brown surface and white interior is striking. The trick here is to cook the scallops 90 percent on the first side, until very well seared, then just give them a quick finish on the other side.
Preheat a gas grill or prepare a fire in a charcoal grill. Place a piastra on the grill to preheat.
Slice the tomatoes creatively (leave very small ones whole, or halve them) and lay out on a platter. Tear the basil leaves over the tomatoes, strewing them about. Set aside.
Season the onion slices on both sides with salt and pepper. Place them on the hot dry piastra and cook, unmoved, for 7 to 10 minutes, until well charred on the first side. Using tongs, carefully turn the slices over and cook for 7 to 10 minutes on the second side, until well charred and softened. Transfer to a plate and let cool slightly, then separate the onion slices into smaller rings and scatter them over the tomatoes. Drizzle the whole mess with 3 to 4 tablespoons of the olive oil.
While the onions cook, carve a checkerboard pattern about 1/4 inch deep into one side of each scallop. Season them all over with salt and pepper, toss them in a bowl with the remaining 2 tablespoons oil, and stir gently to coat.
Place the scallops on the dry clean piastra, design side down, and cook for 5 to 7 minutes, unmoved, until almost cooked (they should be opaque almost all the way through). Flip them over and sear for just 30 seconds, then remove and arrange on the tomato salad.
Sprinkle the tomatoes with the Maldon salt, squeeze the lemon halves over the scallops and tomatoes, and serve.