The night before making the chili: Rinse and cover the black beans with water in a bowl. Soak overnight at room temperature.
Prepare the fresh ingredients (this can be done in advance and kept in the refrigerator): Dice the medium onions, mince the garlic, and slice the mushrooms.
Saute the vegetables (can be done in advance, or directly before on the day of making): On the stove, heat the olive oil over medium heat. Add the onions, garlic, and mushrooms. Stir in the chili powder, chipotle powder, and kosher salt. Saute until the onions are tender, about 5 minutes. Remove from the heat.
About 10 hours before serving, discard the liquid on the beans and place the beans in a large slow cooker. Transfer the onion and mushroom mixture to the slow cooker with the beans and add the vegetable broth and crushed tomatoes. Cook on low for 10 to 12 hours until the beans are tender.
When ready to serve, garnish with sliced green onions and sour cream or cashew cream.