In a large, nonreactive skillet, melt the butter over medium heat.
Add the onions, salt and garlic and cook until they begin to soften, 5-7 minutes.
Reduce the heat to medium-low and cook until the onions take on deep brown caramelization.
Stir occasionally as they cook, and as brown bits accumulate on the bottom of the pan, deglaze with 1 tablespoon of apple cider at a time. Continue cooking until the onions are melty and deep brown, about 50 minutes.
Stir the apple cider vinegar into the caramelized onions and transfer to a bowl.
Add the Greek yogurt, sour cream and parsley and stir to combine well.
Taste and adjust seasoning with salt, pepper and additional apple cider vinegar. Serve with potato chips and crudites.
