Preheat a grill to medium. In a large bowl, combine the creamed corn, onion, breadcrumbs, eggs, baking powder, 1 teaspoon salt and 1/2 teaspoon pepper.
Position a large cast-iron skillet over the grate. Add 1 1/2 tablespoons oil and heat until hot. Working in batches, drop 1/4 cupfuls of the batter onto the skillet, leaving an inch in between. Cover and cook, turning once and pressing gently to flatten, until browned and cooked through, about 8 minutes. Drain on paper towels. Repeat with another 1 1/2 tablespoons oil and the remaining batter.
Increase the heat to high. Brush the tomato slices with the remaining 1 tablespoon oil and season with salt and pepper. Arrange on the grate in a single layer and grill, turning once, until grill marks appear, about 3 minutes.
Transfer the corn fritters to a platter and top with the tomato slices and basil.