Heat oil in a Dutch oven over medium heat. Add onion; cook 4 minutes, stirring frequently. Add chayote, poblano, and garlic; cook for 4 minutes, stirring frequently. Add kabocha squash and next 7 ingredients (through oregano); bring to a boil. Reduce heat, and simmer for 1 hour or until vegetables are tender. Discard oregano sprigs. Stir in 1 teaspoon salt and black pepper. Ladle stew into individual bowls, and serve with lime wedges.