Stir together mashed potatoes, bacon, and green onions. Spread mashed potato mixture onto 4 tortillas, spreading to within 1/2 inch of the edges. Sprinkle shredded Cheddar cheese on top of mashed potatoes and top with remaining tortillas. Lightly butter each side of quesadilla.
Preheat a large skillet over medium heat.
Place a quesadilla into the preheated pan; cook until golden brown and crispy, 2 to 3 minutes per side. Repeat with remaining quesadillas. Serve with salsa and sour cream.
