Preheat oven to 400 degrees F. Arrange purple sweet potato and onion in a single layer on a baking sheet. Bake for 20 minutes; then flip the potatoes, and bake for another 10 to 15 minutes or until sweet potato can be pierced with a fork.
Transfer the baked sweet potato and onion to the bowl of a food processor. Add the tahini, lemon juice, chipotle, smoked paprika and salt. Process until chopped and chunky.
With the processor running, drizzle a few tablespoons of water through the feed tube. Check the consistency, and add more water if needed. Taste and adjust salt and spice as desired.
Chipotle Purple Sweet Potato Dip is best served at room temperature. Store leftovers in the refrigerator for up to 3 days.