Preheat the oven to 475°. Oil the bottom of a very large roasting pan. Arrange the potatoes in a slightly overlapping layer in the pan; drizzle with oil and season with salt. Spread the tomatoes over the potatoes, drizzle with oil and season with salt. Bake in the lower third of the oven for 50 minutes, or until the potatoes are tender and crisp on the bottom.
Meanwhile, in a mortar, pound the garlic, a few cloves at a time, with a little salt until well mashed. Add the chile and pound to a paste. Blend in 1/4 cup of the oil and transfer the paste to a medium bowl. Stir in the cilantro, lemon juice, cumin, paprika and the remaining 1/2 cup of oil. Season with salt. (Alternatively, blend all of the charmoula sauce ingredients in a mini food processor.).
Slash the skin side of the fish fillets 3 times. Transfer the fish fillets, skin side down, to a large rimmed baking sheet and coat the fish with half of the charmoula sauce. Let marinate at room temperature for 30 minutes.
When the potatoes are tender, set the fish fillets, skin side down, on top and season with salt. Bake in the upper third of the oven for about 12 minutes, or until the fish is just cooked through. Serve hot or at room temperature with the remaining charmoula on the side.