Rinse rice in a large sieve under cold water, then drain.
Heat oil in a 5-quart heavy pot over medium heat until hot but not smoking.
Cook onion, stirring occasionally, until golden, about 5 minutes.
Add brown 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 brown rice is tender, 1 to 1 1/4 hours. Drain well in sieve and transfer to a bowl.
While brown 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 loose genmaicha leaves, almonds, salt, and pepper to rice and stir gently to combine.