In sauce pan over medium heat, bring broth to a gentle simmer. In a large heavy saucepan, heat olive oil on medium and add onion. Cook until tender (about 4 min). Add rice and stir until each grain is coated with oil and translucent with a white center. Add broth a cup at a time until it is almost absorbed, reserving the last 1/4 cup. When rice is almost tender to bite and looks creamy, remove from heat and stir in remaining broth. Add butter, salt and pepper to taste.
