Melt butter in a pan. Add onions and cook on med-low heat for 15 minutes, stirring periodically, until soft. Add brown sugar and apple cider vinegar, stir to coat onions and cook for an additional 15 minutes until the onions are golden and sweet. Remove from heat and set aside.
Spread half the onions over one half of a tortilla. Top with half the cheese and half the time. Fold the tortilla over and press gently so that the tortilla gently hugs the filling. Repeat with the second tortilla and the remaining ingredients.
Heat the pan to medium-high heat. Gently place one tortilla in the pan and cook for 2-3 minute until lightly browned. Flip and cook the other side for 2-3 minutes. Repeat with the second tortilla. Cut the tortillas into slices and serve immediately.
Recipe by Port and Fin at https://portandfin.com/sticky-caramelized-onion-smoked-gouda-quesadillas/