Heat oven to 350º F. Cook the rice according to the package directions.
Wrap the tortillas in aluminum foil and heat in oven for 15 minutes.
Meanwhile, sauté the onion in oil over medium heat until softened, about 3 minutes.
Add the rice, beans, and salt and cook for 2 minutes. Stir in the cilantro.
Divide the rice-and-bean mixture among the warm tortillas. Top each with some cheese and spinach and roll. Serve with salsa or sour cream, if desired.