Preheat the oven to 350 degrees F (175 degrees C).

Bring chicken broth to a boil in a medium saucepan.

Mix hot chicken broth, uncooked wild rice, water chestnuts, 2 tablespoons butter, and salt together in a 1 1/2-quart baking dish until well combined. Cover the dish with aluminum foil.

Bake in the preheated oven for 30 minutes.

While rice mixture is baking, melt remaining 2 tablespoons butter in a medium skillet over medium heat. Add mushrooms and onion to the skillet; cook and stir until tender. Set aside.

Remove the dish from the oven; uncover and stir in mushroom-onion mixture until evenly incorporated. Cover the dish and continue baking until liquid has been absorbed, about 30 more minutes.