Prepare fish and vegetables: Preheat oven to 425°F.
Prick each potato once with a fork, then rub potatoes with 1/2 tablespoon oil. Roast on a baking sheet in middle of oven until just tender, about 25 minutes. Cool to room temperature and cut crosswise into 1/4-inch-thick slices. Leave oven on.
Heat remaining 1 1/2 tablespoons oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté bell peppers, stirring, until just tender, 6 to 8 minutes.
Spread potato slices evenly in an oiled 13- by 9- by 2-inch glass baking dish and season with salt and pepper. Top with peppers, then tomatoes and fish, seasoning each layer with salt and pepper. Sprinkle fish with lemon juice.
Make charmoula sauce: Purée all sauce ingredients except oil in a food processor or blender. With motor running, add oil in a slow stream.
Bake fish and vegetables: Pour sauce evenly over fish and bake in middle of oven until fish is just cooked through, 25 to 30 minutes.