Preheat the oven to 375 F.
Add the butter and sliced onions to a large oven-proof pan and saute for 15 minutes, or until almost caramelized.
Add the mushrooms and garlic to the pan. Saute on medium heat until the mushrooms are browned, about 5 minutes.
Add the rice and toss to coat.
Add the beef broth and French onion soup mix. Stir to combine.
Add the thyme to the pan.
Place a lid on the pan and bake in the oven for 25 minutes, or until all the beef broth has been absorbed.
Let sit for 5 minutes then remove the thyme stems.
Fluff the rice with a fork and serve.