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)
1. Heat 2 tbsp. oil in a 12" skillet over medium heat. Add celery, onions, and garlic and cook, stirring occasionally, until soft, about 8 minutes. Transfer celery–onion mixture to a large bowl and set aside.
2. Put potatoes into a 4-qt. saucepan and cover with salted water by 1". Bring to a boil, reduce heat to medium, and simmer until potatoes are tender, about 15 minutes. Drain; transfer half the potatoes to a plate. Set aside to let cool. Transfer remaining potatoes to a bowl and mash with a fork. Transfer mashed potatoes to reserved bowl of onion mixture; set aside to let cool.
3. Season cod with salt and pepper. Heat 2 tbsp. oil in a 12" nonstick skillet over medium heat. Add cod and cook, turning once with a metal spatula, until cooked through, 8–10 minutes. Transfer cod to a plate and let cool. Break cod into 1" chunks and set aside.
4. Add bread crumbs, mayonnaise, herbs, egg yolk, and lemon juice to the potato–onion mixture and stir vigorously to combine. Add the reserved cubed potatoes and the cod and mix gently to combine. Using your hands, divide the mixture into 8 equal portions and form into 3"-wide cakes (use a 3" ring mold if you have one). Transfer cakes to a wax paper–lined baking sheet, cover with plastic wrap, and refrigerate for 30 minutes, until firm. Working in 2 batches, heat 1 tbsp. oil and 2 tbsp. butter in a 12" cast-iron skillet over medium-high heat. Add cod cakes and cook, flipping once, until golden brown, about 8 minutes. Transfer cakes to a serving platter; serve with chowchow, if you like.