Bring pot of water to a boil for the rice. Once boiling, season with salt and pepper and add rice. Cover and boil gently until it is cooked, about 45 minutes. Set aside.
As the rice cooks, prepare the other ingredients. In a pot of cold water, add prepared potatoes and season with salt, bring to a simmer and cook gently until tender, about 20 minutes. Strain water and set aside potatoes.
Render bacon in a large sauté pan over medium heat. Remove bacon once cooked and leave fat behind.
Add onions and mushrooms to the sauté pan and cook in the bacon fat until the mushrooms are brown. Season liberally with salt, pepper, and sage, and remove from heat.
In a large mixing bowl, combine the cooked rice, potatoes, bacon, mushrooms, and onions.
Grease a baking dish with 3 tablespoons of butter, and add the rice mixture. Cover with foil and bake 1 hour.