At least 12 hours, up to one week before making the sandwiches, place matchstick carrots, pepper, onions, lemon peel and red pepper flakes in a quart-sized jar. Combine the water, vinegar, sugar, salt, coriander, and cumin seeds in a small pot. Heat over medium heat until the mixture comes to a boil and sugar/salt as dissolved. Pour over the carrots, cool, and place the lid on the jar. Store in refrigerator.
To assemble sandwiches, spread 1 tablespoon of hummus on all four slices of bread. Layer a handful of sprouts on two of the slices, followed by a layer of the pickled carrot mixture and finally feta. Place the tops on the sandwiches, slice in half, and pack away for lunch.
Notes
+ Pickled recipe lightly adapted from this recipe from Bon Appetit.
+ If you're in need of a hummus recipe, I still love this one from Mark Bittman.