Preheat oven to 350°F, with a large sheet pan placed in the oven while preheating.

Spread the chopped potatoes across the preheated sheet pan and drizzle with enough oil to coat. Sprinkle a generous amount of kosher salt and fresh-cracked pepper and toss. Roast, flipping halfway through. Cook until soft in the middle but crispy on the outside, about 30 minutes.

While potatoes are cooking, slice leek lengthwise and slice thinly into half-moons. Heat a large, ovenproof sauté pan over medium heat. Melt butter, and add leek once butter begins to foam. Cook for a couple of minutes, then add garlic and kale. Continue to cook until kale softens but is not fully wilted. Deglaze the pan with white wine and stir in the heavy cream. Add lemon zest. Reduce heat to a simmer and let the sauce reduce. Taste and season with salt and pepper as needed.

Turn off heat and gently place each fillet into the pan, pushing them down to partially submerge. Transfer the whole pan to the oven to finish cooking.

Braise fish in the sauce until just barely cooked through, about 6–8 minutes. It should flake easily with a fork when done.

Serve the fish fillets and sauce with the crispy potatoes. Sprinkle with fresh dill or parsley to finish.