Cut onion into 1/4-inch thick slices. Cut off stem end of chiles. Cut chiles in half lengthwise and remove seeds and veins. Slice chiles into 1/4-inch wide strips. In a 10-inch skillet, heat oil with chili powder and salt over high heat. Add chiles and onions; saute 2 minutes, stirring frequently. Pour in rice vinegar. Cook and stir 4 minutes or until vinegar evaporates; remove from heat.
Meanwhile, preheat another 10-12-inch skillet. (An iron frying pan or non-stick pan works well.) To make quesadillas one at a time: Place a tortilla in hot skillet. Turn over when tortilla starts to brown on the bottom. Sprinkle 1/2-cup cheese on half the tortilla. Arrange some of the Tangy Chiles and Onions over cheese, then fold tortilla over cheese. Cook until cheese melts; turn over once to brown both sides.
Total preparation and cook time: 20 minutes
Nutrition information per serving: 410 calories, 18 g protein, 30 g carbohydrate, 3 g dietary fiber, 25 g fat, 14 g saturated fat, 50 mg cholesterol, 1130 mg sodium