I got the recipe from The Food & Wine. I added strawberries to the recipe.

ingredients:

4 medium carrots, sliced into thin rounds
4 medium inner celery ribs, thinly sliced on the bias, plus 1/2 cup lightly packed celery leaves
1 cup thinly sliced white onion
handful of strawberries thinly sliced
3/4 cup plus 2 tablespoons distilled white vinegar
2 garlic cloves
2 tablespoons sugar
Kosher salt
Pepper
2 tablespoons extra-virgin olive oil
1/2 cup pumpkin seeds (pepitas)
1 tablespoon harissa
Three 6-ounce romaine hearts, dark outer leaves removed and inner leaves torn into bite-size pieces (10 cups)

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.
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.