Heat olive oil in a skillet and add onion. Cook for 5-8 minutes or until translucent. Add garlic and saute for an additional 30 seconds to release the flavor. Add the mushrooms and cook until mushrooms just begin to soften.
In a small bowl dissolve miso into 1½ cup of vegetable broth and add black pepper. Spray a 8x8 baking dish or similar size with baking spray. Add rice and veggies to baking dish and add the broth. Stir just enough to disperse ingredients in the baking dish. Place into oven to bake.
At 45 minutes, check the water level and doneness of the rice. If the rice is still crunchy it needs to cook longer. If all the water has cooked out at this point add ½ cup of water and return the oven to cook another 15 minutes or until rice is done. After this point, if the water cooks up and the rice is still not done, add no more than ¼ cup of water and repeat as needed until rice is cooked.