Slice the sweet potatoes plank-style, 1/4" thick by 1/2" wide. This gives more surface area to cook them, making them crispy instead of mushy-which tends to happen with this type of high-starch potato.
Soak the cut slices for 30 minutes in very cold water (with ice or in fridge) to remove as much starch as possible from their surface.. Drain and pat dry.
Toss the sweet potatoes with the oil, salt, and pepper, and spread in an even layer on a preheated baking pan or a baking stone (for crispier fries).
Bake at 400°F for about 40 minutes, turning a few times to ensure even browning.
Remove from the oven and sprinkle with a bit of coarse sea salt if desired, and serve with mayo.
To prepare spicy mayo, whisk all ingredients together until smooth.