While the water is heating, trim the ends off the asparagus and place them in a single layer in a vacuum bag. Add the butter and zest to the pouch and vacuum seal. Add to heated water and cook for 14 minutes.
Remove bag from water bath after 14 minutes, cut open pouch and plate. Add a little lemon juice and salt to taste and top with crispy onion and garlic. ORToss asparagus on a hot grill for 30 seconds to a minute, depending on your grill. This gives them a nice char, without over cooking. Then add the citrus juice, salt and toppings, as above.