In a large, oven-proof pot, melt butter; add onion and simmer until transparent. Add rice and cook until golden brown. Add consomme and mushrooms with the liquid, and give it a good stir. Cover and bake at 325° for 1-1/2 hours. (I take it out and stir it about every 1/2 hour)
Remove from oven, stir it one last time and serve.
I've also made this the night before a big dinner and heated it in the crock pot on the low setting for a couple of hours, until it's heated through.
2Did I mention it tastes better the next day? If there are leftovers I cut up pieces of leftover beef roast and add it to the rice, heat it, and serve it for lunch or dinner. Yum!