In a 2- to 3-quart nonstick pan over medium-high heat, mix onion, garlic, and olive oil. Add rice and stir until onion is barely limp, 2 to 3 minutes.
Stir in 1 cup spinach broth and bring to a simmer, then reduce heat to maintain simmer and stir until liquid is absorbed, 2 to 3 minutes. Add 2 more cups broth, 1/2 cup at a time, stirring after each addition until liquid is absorbed and rice is tender but still has a slight bite, 20 to 25 minutes total. If rice isn't quite done, add more broth and stir until risotto is creamy.
Crumble cheese into risotto and stir until blended. Season with salt and pepper to taste. Garnish with lemon peel.