Preheat the oven to 350. Saute the onions in the oil for about 8 minutes, until translucent. Stir in the tomatoes, corn, black beans, lime juice, salt, and pepper and continue to saute for another 5 to 10 minutes, until just heated through. Meanwhile, in another saucepan, blanch the greens in boiling water to cover for 1 to 3 minutes, until just wilted but still bright green. Drain immediately and set aside. Prepare an 8x8 inch casserole dish or baking pan with a very light coating of oil or cooking spray. Spread half of the crushed tortilla chips on the bottom. Spoon the sauteed vegetables over the tortilla chips and sprinkle on about two-thirds of the grated Cheddar cheese. Arrange the greens evenly over the cheese and spoon on half of the salsa. Finish with the rest of the tortilla chips and top with the remaining salsa and Cheddar. Bake for about 35 to 40 minutes, until the cheese is bubbling and beginning to brown.