Season ground meat with all the ingredients listed with the exception of the salt. Mix well and set aside for 15 minutes. Mix in the salt and make 8 patties to fit the sweet potatoes. Heat a large nonstick skillet on med-high heat with some olive oil. When hot cook burger 5-6 minutes on each side. To Plate: Top one sweet potato round with lettuce, then 1 patty, add garnish, marinated onion, tzatziki sauce, and cover with another sweet potato round. Repeat with rest and enjoy it.