In a small mixing bowl, combine the non-fat Greek yogurt and dry ranch seasoning until smooth to create a high-protein ranch spread.
Add the shredded chicken breast and chopped green onions to the ranch spread, tossing until the chicken is evenly coated.
Place the tortilla on a flat surface and spread the chicken mixture across one half of the tortilla.
Evenly distribute the shredded cheddar cheese over the chicken mixture.
Fold the empty half of the tortilla over the filling to create a half-moon shape.
Preheat a non-stick skillet over medium-high heat and lightly coat with olive oil or non-stick spray.
Place the quesadilla in the skillet and cook for 3 to 4 minutes on the first side, or until the tortilla is crisp and golden brown.
Carefully flip the quesadilla and cook for an additional 3 minutes until the cheese is fully melted and the second side is golden.
Remove from heat, allow to rest for 1 minute for the cheese to set, and slice into three triangles.