In a large, heavy pot, heat the oil over medium-high heat. Add the onions, bell peppers, garlic, and serrano peppers, and cook, stirring, until soft, about 3 minutes. Add the corn, and mushrooms, and cook, stirring, until soft and the vegetables give off their liquid and start to brown around the edges, about 6 minutes. Add the chili powder, cumin, salt and cayenne, and cook, stirring, until fragrant, about 30 seconds. Add the tomatoes and stir well. Add the beans, tomato sauce, and vegetable stock, stir well, and bring to a boil. Reduce the heat to medium-low and simmer, stirring occasionally, for about 20 minutes.
Remove from the heat and stir in the cilantro. Adjust the seasoning, to taste.
To serve, place 1/4 cup of brown rice in the bottom of each bowl. Ladle the chili into the bowls over the rice. Top each serving with a dollop of sour cream and spoonful of avocado.