How to Make It
In a heatproof medium bowl, combine the carrots, celery ribs and onion. In a medium saucepan, combine 3/4 cup of the vinegar with the garlic, sugar, 1 1/2 cups water and 1 1/2 tablespoons of salt and cook over moderate heat until the sugar dissolves, about 3 minutes. Pour the hot brine over the vegetables and let stand until cool, about 30 minutes. Drain the vegetables and refrigerate until just chilled, about 15 minutes; discard the garlic. Reserve the brine for another use.
Meanwhile, in a small skillet, heat 1/2 tablespoon of the olive oil. Add the pumpkin seeds and a pinch each of salt and pepper and cook over moderate heat, stirring, until lightly browned, about 2 minutes. Let cool.
In a serving bowl, whisk the harissa with the remaining 3 tablespoons of olive oil and 2 tablespoons of white vinegar; season with salt and pepper. Add the romaine, drained pickled vegetables and celery leaves and toss well; season with salt and pepper and toss again. Sprinkle the toasted pumpkin seeds on top and serve.