Remove the bread and sweet potatoes from oven and peel the sweet potatoes when they are cool enough to handle, about 10 minutes after cooking. Discard peels and mash the inner sweet potato flesh in a medium sized bowl.
Spoon the sweet potato flesh on the top of the toasted bread slices then bake for another 10 minutes or until the sweet potato topping is golden brown and the bread is toasty. Remove from the oven and top each piece with onion, a drizzle of lemon juice, basil, sea salt, pepper and lemon zest. Serve warm.