Cod Cakes

Name: Cod Cakes
Ingredients: 6 tbsp. extra-virgin olive oil; 2 ribs celery, finely chopped; 1 medium onion, finely chopped; 1 clove garlic, finely chopped; 2 russet potatoes (about 1 lb.), peeled and cut into 1⁄4″ cubes; Kosher salt, to taste; 1 lb. boneless skinless cod filets; Freshly ground black pepper, to taste; 1⁄2 cup dried bread crumbs; 1⁄4 cup mayonnaise; 2 tbsp. finely chopped fresh dill; 2 tbsp. finely chopped flat-leaf parsley; 1 egg yolk, beaten; 1 tbsp. lemon juice; 4 tbsp. unsalted butter; Chowchow, for serving (optional)