How to Make It
Make the pickled celery In a small saucepan, combine the vinegar, water, salt and sugar and bring to a boil, stirring to dissolve the salt and sugar. Let cool completely, then transfer to a medium bowl. Add the celery, cover and refrigerate for at least 4 hours or overnight. Drain before using.
Make the tartines In a large skillet or on a griddle, melt half of the butter. Add the bread and cook over moderate heat until golden on the bottom, about 2 minutes. Flip the bread, add the remaining butter and cook until the bread is toasted, 1 to 2 minutes more. Transfer to a work surface.
Spread the yogurt on the toasts. Top with the sliced cucumber and season with salt. Top the tartines with the chicken and half of the drained pickled celery; reserve the remaining celery for another use. Garnish with small dill sprigs and serve right away.