Lay the salmon filet on a sheet of aluminum foil. Fold and seal the foil together to enclose the salmon inside. Bake in preheated oven for 30 minutes or until the salmon flakes easily with a fork.
While the salmon is cooking, bring the 1/2 cup of water to a boil and add the rice. Cover, remove the pan from the heat, and let sit for 5 minutes or until all the water has been absorbed.
Wrap the tortillas tightly in aluminum foil. During the last few minutes of the salmon's cook time, put the wrapped tortillas in the oven to warm. When the salmon is done, remove it from the foil and cut it into 4 equal pieces.
Remove a warm tortilla from the oven, one at a time, filling each one with a piece of salmon. Break the piece of salmon up and place it on the tortilla. Add an 1/8 cup of cooked rice, 1/4 cup of cheese, and 1 tablespoon of red onion. Drizzle 1 tablespoon of ranch dressing over the top. Wrap the tortilla up and serve warm.