Preheat the oven to 425 degrees F. In a large mixing bowl, toss the sweet potatoes, leeks, olive oil, salt, cumin and paprika until well mixed. Arrange in an even layer on two parchment paper-lined baking sheets. Roast in the oven until golden brown and caramelized, about 25 minutes, swapping the pans from top to bottom rack halfway through.
Wrap the tortillas in foil and warm in the oven during the last 5 minutes of cooking.
Meanwhile, make the tahini sauce (recipe follows).
Schmear the tortillas with the refried black beans, top with the sweet potatoes and serve with a drizzle of chipotle tahini sauce and lime wedges on the side.
To remove grit from the leeks, halve them lengthwise and rinse them under cold water, fanning out the layers. The proceed and thinly sliced into half-moons.