Heat oven to 400° F.
Heat 2 tablespoons of the oil in a large ovenproof skillet over medium-high heat. Add the sausage and brown on both sides. Add the potatoes and cook, stirring occasionally, for 10 minutes. Add the leeks, broth, saffron, 3/4 teaspoon of the salt, and 1/8 teaspoon of the pepper. Bring to a boil.
Meanwhile, rinse the cod and pat it dry with paper towels. Season with the remaining salt and pepper. Place it on top of the potatoes and drizzle with the remaining oil. Bake until the cod is the same color throughout and flakes easily, about 20 minutes. Sprinkle with the parsley and transfer to individual plates.
Recommended: Large ovenproof skillet