Serve with sour creamlime, cilantro, avocado, etc.
In a large pan over medium heat, heat the oil and cook the onion until soft and tender, about 5 minutes. Add the garlic and cook for another minute or so until fragrant. Add the chipotle peppers and the diced tomatoes with their liquid. Stir frequently and cook until the sauce thickens and the liquid has mostly evaporated, about 20 minutes. Stir in the chicken, green onions, and cilantro. Reduce heat to low just to keep mixture warm.
Heat a clean grill on medium heat. I like to put a touch of oil in the pan to make the tortilla extra crispy, but that's not necessary and is a matter of preference. Place tortilla on hot grill, cover with a layer of cheese, and then spread some of the chicken chipotle mixture over half. Once the cheese has melted, fold the tortilla over to make your quesadilla. Repeat with remaining tortillas. You can keep them all warm in the oven until you're ready to serve. Allow them to rest for a couple minutes before slicing to ensure everything doesn't ooze out.
Serve with fresh lime slices, sour cream, avocado slices, extra cilantro, and an ice cold beer or a margarita (or a beergarita).