1. Heat 1 tbsp olive oil in a large skillet over medium heat until shimmering. Add the onion to the pan and saute for three minutes, stirring occasionally.
2. While the onion is sauteing, slice the sausage into ¼” slices.
3. Stir the sausage into the skillet with the onion. Continue to saute over medium heat for another ten minutes (until lightly browned), stirring occasionally. Remove sausage and drippings from pan.
4. Reduce heat to medium. Place one tortilla in the pan. Top with ¼ each of cheese, sausage, and onions. Cook quesadilla for 1-2 minutes or until the bottom is crisp and lightly browned and cheese is melted. Remove quesadilla from pan and repeat with remaining tortillas.
5. Top each tortilla with guacamole, sausage, and onions and serve with salsa, queso, and chopped cilantro if desired.