Cover a 9x13 baking dish with foil and drizzle with olive oil. Arrange the potatoes and leeks cut-side up in the pan. Season with salt and 1/2 teaspoon piment d'espelette. Dot the veggies with the butter and sprinkle with the garlic and lemon zest. Cover the pan with foil and bake in the oven until the potatoes are tender, about 25 minutes.
Meanwhile, in a small mixing bowl, combine the lemon juice, olives, celery, and parsley. Season lightly with salt and toss to combine. Set aside.
After the potatoes have cooking for 15 minutes, heat 1 tablespoon olive oil a large non-stick skillet. Season the hake on both sides with salt and the remaining piment d'Espelette. Sear the fish over high heat until nicely browned, 3 minutes.Carefully flip and cook 2 to 3 minutes more.
Divide the leeks and potatoes between two plates and top with the seared hake. Garnish with the celery salad and serve immediately.
Notes
If you can't find piment d'espelette you can substitute smoked paprika with a dash of cayenne.