Preheat oven to 400 degrees F (200 degrees C). Drizzle a large oven-safe pan with oil.
Rinse trout and pat dry with a paper towel. Place on a flat work surface. Cut trout from one side through the middle horizontally to within one-half inch of the other side. Open the two sides and spread them out like an open book; lay onto the prepared pan.
Place tomatoes, onion, lemon juice, cilantro, garlic, coriander, cumin, salt, pepper, and cinnamon in a food processor. Pulse until chopped fine but not pureed. Pour 1/2 the mixture on top of 1/2 of the trout; fold the other side of trout on top. Spoon the remaining 1/2 of the mixture on and around the sealed trout. Add water to the pan; arrange potatoes around the trout. Cover pan with aluminum foil.
Bake in the preheated oven until trout flakes easily with a fork, about 45 minutes.