How to Make It
In a small bowl, combine the tomatoes, shallot and balsamic vinegar. In another small bowl, whisk together the lemon juice, cracked black pepper and salt. Gradually whisk in 1/3 cup of the olive oil.
In a large nonstick skillet, heat the vegetable oil. Add the onion slices in a single layer and cook over moderately high heat until golden, about 3 minutes per side. Transfer the onions to a plate and separate into rings.
In a large bowl, thoroughly toss the frisée with two -thirds of the dressing. Mound the salad on 6 plates and top with the onion rings.
Light a grill or heat a grill pan over moderately high heat. In a bowl, toss the scallops with the remaining 1 tablespoon olive oil and season them with salt and ground pepper. Grill the scallops until golden and just cooked though, about 1 1/2 minutes per side. Arrange 3 scallops around the edge of each salad and spoon the diced tomatoes on top. Drizzle the remaining dressing over the scallops and serve at once.