Rinse the fish fillets and set them aside. Coat a casserole dish with oil. Arrange the onions and carrots in a layer to cover the bottom of the dish and place the fillets on top. Lightly sprinkle them with salt.
In a blender or food processor, combine the cilantro, garlic, ginger, cumin, lemon juice, chile, and half of the chopped tomatoes and puree to form a smooth sauce. Add salt to taste. Pour the sauce evenly over the fish and vegetables and top with the remaining chopped tomatoes.
Cover tightly with foil and bake for 20 to 25 minutes, until the fish flakes easily with a fork. Serve immediately on couscous.