To prepare the slaw, toss asparagus, cabbage, feta and spring onions in a large bowl and set aside.
Add mayonnaise, apple cider vinegar, sugar, pepper and hot sauce (if using) to a small bowl and mix until creamy. Reserve a few tablespoons of the dressing for the burger buns and pour the remainder over the slaw and mix well. Cover and refrigerate until ready to use.
For the burgers, add everything to a large bowl and mix with your hands until well combined (do not over mix). Form into four equal patties.
Heat oil in a large cast iron pan over medium high heat. Add two patties at a time to the pan and cook until the underside has browned and developed a nice crust - about five to six minutes. Flip the burgers and allow the other side to do the same. This will give you a medium doneness. If you prefer your burgers cooked through, lower the temperature to medium low, cover and continue to cook for an additional four to five minutes.
To assemble, slather a bit of the dressing on the bottom buns, top with Boston Bibb lettuce, a burger and a handful of the slaw. Enjoy!