Preheat oven to 325 degrees
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.