Rinse rice in a large sieve under cold water, then drain. Heat oil in a 5-quart heavy pot over moderate heat until hot but not smoking, then cook onion, stirring occasionally, until golden, about 5 minutes. Add rice and cook, stirring, until fragrant, about 3 minutes. Stir in broth and water and bring to a boil. Reduce heat to low and simmer, covered, until rice is tender (grains will split open), 1 to 1 1/4 hours. Drain well in sieve and transfer to a bowl.
While rice is cooking, melt butter in a 10-inch skillet over moderate heat until foam subsides, then cook almonds, stirring, until golden, about 3 minutes.
Add almonds, salt, and pepper to rice and stir gently to combine.
