In a large bowl, combine mashed potatoes, 1/2 cup tomatillo salsa and green onions. Top one half of each tortilla with half the potato mixture; sprinkle with 1/2 cup cheese. Fold tortilla to close. Lightly butter top and bottom of quesadillas. , In a large cast-iron skillet or griddle, in batches, cook quesadillas over medium heat until golden brown and heated through, 2-3 minutes on each side. Cut into wedges. Serve with sour cream and remaining 1/2 cup tomatillo salsa. If desired, sprinkle with additional green onions.