1. In a medium pot, bring 8 cups water to a simmer; cover and reduce heat to low. In a heavy 4-qt saucepan, heat oil over medium heat. Add onion and 1/2 tsp salt; cook, stirring, until onion is lightly golden, about 5 minutes. Add rice, increase heat to high and cook, stirring constantly, until rice is lightly toasted (not browned) and makes a clicking sound in pan, about 2 minutes. Add wine and stir until evaporated and rice is dry, about 2 minutes.
2. Ladle enough hot water into pan to just cover rice, about 2 cups. Cook, stirring frequently, until almost all of water is absorbed, 1 to 2 minutes. Add more hot water to just cover rice; stir in spinach and remaining 1/2 tsp salt. Cook, stirring frequently, until almost all of water is absorbed, 3 to 5 minutes; add more hot water to just cover. Repeat until a total of 6 cups water have been absorbed, 15 to 20 min. Taste for texture, adding hot water as needed. When risotto is al dente and creamy, remove from heat.
3. Stir in butter and cheese; salt and pepper to taste. Serve immediately with more cheese, if desired. Makes 6 servings.