Position a rack in the upper third of the oven and put a baking sheet on the rack. Preheat the oven to 450 degrees F for at least 10 minutes. Halve the potatoes lengthwise and toss them with the olive oil in a large bowl. Carefully remove the baking sheet from the oven. Using tongs, arrange the potatoes cut-side down on the hot baking sheet. Sprinkle with the thyme, celery leaves, bell pepper strips, half of the lemon juice and 2 teaspoons salt. Roast until the potatoes are tender and golden, 25 to 35 minutes.
Meanwhile, combine the sliced celery and scallions in a small bowl. Whisk the mayonnaise, hot sauce and the remaining lemon juice in another small bowl.
Use a spatula to carefully transfer the potatoes to a cutting board. Cut each potato into 2 or 3 pieces. Arrange the potatoes by color on a platter and top with the red peppers and the celery-scallion mixture. Serve with the spicy mayonnaise.