1. For the crab cakes: Heat a small skillet over medium-low heat with 1 teaspoon butter. Add the celery and scallion and cook until just translucent, about 3 minutes. Season with a pinch of salt and a little pepper. Cool slightly.
2. Put vegetables in a large bowl along with 1/4 cup of the crushed crackers, the mayonnaise, horseradish and lemon zest. Mix everything together, season well with salt and pepper. Gently fold in the crabmeat taking care to keep some of the lumps intact. Divide the mixture into 4 (1/2-inch) patties. Put the remaining cracker crumbs in a shallow dish and coat the patties evenly with the crumbs, cover, and refrigerate for at least 30 minutes to allow the crab cakes to set up.
3. For the sauce: Whisk all the ingredients in a bowl, season with pepper and set aside.
4. Melt the remaining 2 tablespoons butter in a large non-stick skillet over medium-high heat. Cook the crab cakes until golden brown and heated through, turning once, about 2 minutes per side. Serve crab cakes with the sauce garnished with more chopped celery and tender celery leaves, as desired.
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