In a 4-quart saucepan, warm olive oil over medium heat. Add onion, mushrooms, and celery. Cook and stir for 5–7 minutes, until all vegetables are soft, but not brown.
Stir in the chicken broth, brown rice, parsley, salt, and pepper. Cover. Bring to a boil over high heat.
Reduce heat to medium. Cook according to brown rice package directions, about 5–10 minutes. Drain off any excess liquid. Fluff with a fork. Serve immediately.