Scrub and peel the sweet potato. Using a large, sharp knife, slice the potato lengthwise into ¼-⅓ inch oval slices. Very lightly spray slices with cooking spray.
For a regular toaster: place potato slices into bread slots and toast on the highest setting until slightly browned on the outside and soft on the inside. Most regular toasters will take 2-3 rounds. For a toaster oven: place potato slices directly onto the rack and toast on high approximately 10-15 minutes.
Use a paper towel to dry sirloin and season with kosher salt on each side. Heat 1-2 Tbsps cooking oil in a skillet, and sear steak on each side 3-4 minutes for medium-rare.
Remove from skillet and rest 5 minutes before slicing. Thinly slice steak against the grain and top each toast with 2-3 slices and other toppings below
Mix mayo and basil.
Slather sweet potato toast with basil mayo and layer lettuce, tomato slices, and steak slices. Top with dollops of basil mayo and small basil leaves.