Mix together the mashed potatoes, bacon, and spring onions. Spread the mashed potato mixture onto one side of half of the tortillas, spreading to within 1cm of the edges. Sprinkle the grated cheese on top of the mashed potatoes and top with the remaining tortillas. Lightly butter each side of the quesadillas.
Preheat a large frypan over medium heat. Place a quesadilla into the preheated pan, and cook until golden brown and crispy, then turn over, and continue cooking on the other side until browned, 2 to 3 minutes per side. Repeat with remaining quesadillas. Serve with salsa and sour cream.