Add 3 cloves minced garlic to 1/4 cup mayo, stir, and leave aside to use for the dressing.
Combine parsley, 3 cloves garlic, shallot, cumin, coriander, salt, and pepper in a food processor and process until finely minced.
Place lamb mince in a mixing bowl, then add the finely minced parsley, garlic, shallot, and spices to the lamb mince. Add the lightly beaten egg, and mix together by hand until gently combined. Shape into lamb burgers using 1/3 of the mince, spices and herbs, to make a total of 3 lamb burgers.
Heat up a skillet to medium-high. Spread a small amount of spreadable butter on both sides of the hamburger buns, and lightly toast for about 1-2 min each side. Leave aside.
After toasting the hamburger buns, add the lamb burgers to the skillet, and fry each side for approximately 4-5 minutes for medium. Once both sides are fried, remove from heat and allow to rest for 2 minutes.
Spread your garlic mayo on both cuts sides of the hamburger buns. Layer the bottom with thinly sliced red onion and cucumbers, and then the lamb burger. Slather with more garlic mayo, then top with spinach, and feta cheese.