Preheat the oven to 400 degrees. Set aside a ¼ cup of the onions. Place the remaining sliced onions and carrots on a cookie sheet and drizzle liberally with olive oil. Season with salt and pepper and toss to coat. Put the cookie sheet in the oven, and cook until the carrots and onions are tender and browned on the edges, about 40 minutes.
Meanwhile, cook the brown rice according to package directions.
In a saucepan over medium heat, cook the ¼ cup of onions in a tablespoon of olive oil. Let the onion soften, then add the 2 cans of beans and about a half cup of water. Bring to a boil, then reduce heat to low. Cook for 10 minutes, then set aside to cool off a little bit.
In a separate pan, cook the mushrooms in olive oil over medium heat. (You could also roast them in the oven, but for some reason I prefer the flavor of sauteeing them). Cook until brown and a little bit fried in some parts. Add salt and pepper.
Once the beans have cooled a bit, use an immersion blender to puree them. You can make them super smooth, or leave some whole beans in there, it's up to you.
To serve, put cooked brown rice in each bowl and top with black beans, roasted carrots and onions, and mushrooms. Sprinkle chopped green onion on top. Season with extra salt and pepper if desired.