Preheat oven to 375 degrees. Lightly oil a 13 x 9-inch baking dish.
Heat 2 tablespoons oil over medium-high heat in a large skillet. Add the onion, reduce heat to medium-low and cook 5-7 minutes, stirring often.
Add the green and red bell pepper. Cook 2-3 minutes then add the yellow squash. Cook 4-5 minutes or until the yellow squash is tender.
Add the garlic and cook 15 seconds or until fragrant.
Add black beans and remove from heat.
Heat remaining 1/4 cup oil in another skillet over medium-high heat. Add the tortillas and cook until crispy and light gold in color. Stir frequently so they brown and crisp evenly. Drain on paper towels.
Stir tortilla squares into black bean mixture.
Add the enchilada sauce and half the cheese.
Transfer to the prepared baking dish. Top with remaining cheese.
Cover with aluminum foil. Bake for 30-35 minutes. Remove foil and continue baking until cheese is melted and casserole is bubbling.
Serve with avocado, cilantro and lime wedges.