Combine water and quinoa in a Dutch oven; bring to a boil. Cover, reduce heat, and simmer 20 minutes. Remove from heat. Drain in a sieve over a bowl, reserving cooking liquid; add enough water to cooking liquid to measure 6 cups. Set quinoa aside.
Heat oil in pan over medium heat. Add jalapeño and garlic; cook 30 seconds. Stir in potato, salt, cumin, and black pepper; cook 5 minutes, stirring frequently. Stir in 6 cups cooking liquid, quinoa, and 1/3 cup onions; bring to a boil. Reduce heat, and simmer 10 minutes or until potato is tender. Stir in 1/3 cup onions and spinach; cook 3 minutes. Remove from heat. Stir in cheese and cilantro.