Slice sweet potatoes lengthwise into 1/4 inch slices. Add to the toaster like you would bread and toast on high. Toast them 2-3 times through until there is a little bit a browning on both sides.
In a medium bowl combine beef, fennel, sage and salt and mix with hands to combine. Divide into 4 oval shaped patties (to fit on a sweet potato slice) and cook on a skillet over medium high heat for 3 minutes on each side.
Once patties are done, break eggs (separately) into the pan and fry until your desired egg style is reached. (runny yolk or cooked through!)
Build burger with some ketchup, burger patty, egg and bacon!
You need a sharp knife to slice easily through a sweet potato! I LOVE this ZHEN knife!