In a small saucepan, heat broth.
Combine olive oil and butter in a saucepan over medium heat, melting the butter. Sauté onion until softened, then add rice and coat with oil. Add hot broth one ladle at a time, stirring constantly. Continue to add broth until rice is cooked but not overly soft. Stir in Parmesan cheese, then season with salt and pepper to taste.
You can add spinach, asparagus, mushrooms, leeks, or whatever else you like — simply sauté more substantial vegetables with the onion or add toward the end of cooking, allowing delicate additions like spinach to wilt before serving.