Preheat oven to 425°. Line a rimmed baking sheet with parchment. Prick sweet potatoes with a fork, and toss on the baking sheet with 1 tablespoon olive oil. Roast until tender, about 45 minutes (turning halfway through).
Remove sweet potatoes from the oven, and let cool slightly. When cool enough to handle, turn the oven to broil. Remove parchment from the baking sheet. Break sweet potatoes into large pieces using 2 spoons. Spread sweet potatoes evenly on the baking sheet, flesh sides up. Season with salt, and drizzle with the remaining tablespoon olive oil. Broil until charred in spots, about 5 more minutes.
Meanwhile, in a small saucepan, combine the grapeseed oil, peppercorns, cinnamon stick, and star anise. Cook over medium heat until oil is hot and fragrant, about 5 minutes. Remove from the heat, and add the red pepper flakes. Pour chile oil into a small heatproof bowl, and let sit until ready to use, at least 10 minutes.
In another small bowl, whisk the tahini with 1 tablespoon lemon juice and 2 tablespoons water. Add more water if necessary, until the consistency is good for drizzling. Season with salt.
In a medium bowl, toss the fennel and onion with the remaining 2 teaspoons lemon juice. Season with salt.
To serve, strain the chile oil through a fine-mesh sieve, discarding solids. Arrange cooked sweet potatoes on a platter. Drizzle with the chile oil and tahini sauce. Top with the fennel salad, basil, and sesame seeds.
Recipe by Laura Rege.