Position a rack in the upper third of the oven and preheat to 450 degrees. On a rimmed baking sheet, toss the potatoes with 2 tablespoons olive oil; season with salt and pepper. Arrange in a single layer and bake until golden, about 20 minutes.
Meanwhile, heat a heavy skillet over medium-high heat. Add the remaining 1/2 tablespoon olive oil and the ground beef and cook, breaking up the meat with a wooden spoon, until browned, 2 to 3 minutes. Mix in the beans, taco seasoning and 2 tablespoons water. Lower the heat and simmer, stirring, until heated through, about 10 minutes.
Transfer the potatoes to an ovenproof platter. Sprinkle half of the cheddar on top, spoon on the meat mixture, then top with the tomatoes and remaining cheddar. Place in the oven and turn off the heat. Let stand until the cheese is melted, 2 to 3 minutes. Serve with the sour cream and lettuce.