In a small saucepan over medium heat, bring stock to a simmer; maintain over low heat.

In a large, heavy saucepan over medium heat, warm oil. Add onion; saute until softened, about 4 min. Add spinach, reduce heat to low, cover and cook until tender, about 5 min. Using a slotted spoon, transfer to a bowl; set aside.

Add rice to pan; stir until well coated with oil and translucent with a white dot in the center, about 3 min. Add wine and stir until absorbed. Add stock a ladleful at a time, stirring frequently after each addition. Wait until stock is almost completely absorbed before adding more. Reserve 1/4 cup stock to add at the end.

When rice is almost tender to the bite but slightly firm in the center and looks creamy, after about 18 min., add spinach mixture to pan and add a ladleful of stock. Cook, stirring occasionally, until spinach mixture is heated through and rice is al dente, 2-3 min.

Remove from heat; stir in butter, cheese and reserved 1/4 cup stock. Season with nutmeg, salt and pepper.