Step 1Heat oil in (3-quart) non-aluminum pot over medium-high heat. When hot, add garlic, onions, zucchini, cumin, chile powder, chipotle puree, oregano, salt and broth. Stir well. Cook until mixture is softened, stirring often, about 7 minutes.
Step 2Puree tomatoes and their liquid with tomato paste and add to pot, along with corn and black beans. Bring to boil, then reduce heat and simmer, partially covered, stirring occasionally, until mixture reaches a chili consistency, about 1 hour. Taste and adjust chipotle. (Can be made ahead and refrigerated 3 days or frozen 1 month. To serve, gently reheat, adding water if chili is too thick. Taste and adjust seasonings.) Serve hot, garnished with cilantro, passing sour cream and sliced green onions separately.