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Preheat oven to 350ºF.
Spread the tortillas evenly onto a large baking sheet.
Bake for 10-15 minutes until dry and crisp.
When cool enough to handle, crumble into small pieces; set aside.
Meanwhile, in a heavy skillet, heat the oil to medium-high.
Sauté the onions until translucent.
Add the zucchini and green peppers (if using canned green chilies, they can be stirred in later - no need to sauté), then continue to sauté the vegetables until the zucchini and peppers are just tender.
Add the garlic; sauté 1 to 2 more minutes.
Stir in the beans, tomatoes and seasonings, bring to a boil, then reduce heat to medium-low and cook gently for 10 minutes.
Lightly spray a 9x13-inch, non-aluminum baking dish or pan with no-stick cooking spray.
Layer half the tortilla chips, half of the tomato-bean sauce and half of the cheese.
Repeat layers.
Bake 25 to 30 minutes, or until bubbly hot throughout.
Let stand 5 to 10 minutes.
Cut into squares.
Serve immediately, garnished with sour cream and fresh cilantro, if desired.