Heat butter and oil in a large saucepan over medium-high heat. Add onion; saute until softened, 3 to 4 minutes. Add rice; stir to coat. (Can turn off heat and let stand several hours.)
Add broth, cover, and bring to a simmer. Reduce heat to low and simmer, stirring occasionally, until stock is absorbed and rice is creamy and soft with a little chew at the center, about 12 minutes. Stir in cheese and pepper; add salt, if necessary. Serve hot.