Rinse and dry the spinach; slice the leaves, a handful at a time, into 1/2-inch wide strips.
Heat the water in a large pot almost to the boil.
Cover, and keep it very hot on the stove, near the risotto pan.
In a heavy 10-inch saucepan, add the olive oil, onions, and 1/2 teaspoon salt; set it over medium heat, and stir well.
Cook, stirring frequently until the onions are wilted and starting to color.
Pour in the rice all at once, increase the heat, and stir continuously for about 2 minutes, until the rice grains are toasted (not browned) and make a clicking sound in the pan.
Pour in the wine, and keep stirring, all around the pan, until it has evaporated and the rice is dry.
Ladle in 2 cups of hot water, enough to cover the rice; cook for a minute or two, stirring, then pile the shredded spinach on top of the rice, and stir steadily as the spinach wilts and the rice gradually absorbs almost all of the moisture, 5 minutes or more.
When you can see the bottom of the saucepan as you stir, ladle in more water to cover the rice, and stir in the remaining 1/2 teaspoon salt.
Cook, stirring frequently, as the risotto develops its creamy suspension.
Again, when the liquid is almost completely absorbed, ladle in another cup or so of water.
After the risotto has cooked for 15-20 minutes and incorporated 6 cups of water, taste; add more salt or more hot water as needed.
When done al dente and creamy, turn off the heat.
Drop in the butter pieces, stir vigorously, then beat in about 1/2 cup of grated cheese, and grind black pepper generously on top.