Put sliced carrots and shallots in a bowl and set aside.
Put apple cider vinegar, brown sugar and salt in a pot and heat until sugar has dissolved. Turn the heat off and pour the liquid over the carrots and shallots. Let cool to room temperature and drain.
Put all the ingredients for the dressing in a bowl, break the goat cheese and mix well. Season with salt and pepper and set aside.
Putting the sandwich together:
Grab 2 slices of bread and spread a generous amount of goat cheese mixture on both slices.
Top with pickled carrots and shallots, a layer of baby spinach and alfalfa sprouts.
Spread mashed avocado on the remaining two slices of bread and lay each slice on top of each sandwich. Serve immediately.