Heat the oil in a medium, heavy-bottomed pot set over medium to medium-high heat. Add the onion, poblano pepper and garlic and cook 4 to 5 minutes. Mix in the chili powder, cumin and oregano and cook 1 minute more. Mix in the diced tomatoes, tomato sauce, stock, squash, black beans, corn and hot sauce.
Bring mixture to a gentle simmer, adjusting the heat as needed to maintain that gentle simmer. Partially cover the stew (do not completely cover; steam needs to escape). Gently simmer 30 to 35 minutes, or until the squash is tender and stew is rich and flavourful. Taste the stew and season with salt and pepper, if needed. Spoon stew into serving bowls, top with a dollop of yogurt, if using, sprinkle with green onion, and enjoy.
Note: Poblano peppers are mildly spicy and have a richer flavour than green bell peppers. They are available in the produce department.