Try using a pressure cooker to caramelize the onions; takes less time with great results!
Slice sweet onions and place in a large heavy bottom saute pan with ghee.
Cook on medium low heat stirring occasionally until onions are soft and caramelized. This can be made a day in advance.
To assemble tart, unroll puff pastry and seal seams together making one large rectangle.
Brush pastry with egg wash and sprinkle with swiss cheese and carmelized onions.
Lay asparagus over toppings and bake in a 400F oven for 40 min or until pastry is browned and asparagus is fork tender.
Whisk mustard and vinegar together and drizzle on tart before serving.