Preheat oven to 400°. Whisk 2/3 cup of oil, 2/3 cup Harissa, 1 tablespoon vinegar in a large bowl to combine. Slice sweet potatoes crosswise on a mandoline 1/8 inch thick.

Add to bowl with mixture and toss to coat; season with salt.

Arrange sweet potato so they are standing upright in concentric circles in a 2 quart baking dish packing tightly.

Roast, brushing any accumulated mixture in dish on to sweet potatoes every 10 to 15 minutes until soft and starting to brown on top, about one hour.

Meanwhile, Toss pistachios, sesame seeds, and fennel seeds on a small baking sheet. Toast alongside sweet potatoes until golden brown, about 40 minutes. Let cool, then transfer to spice mill or mortar and pestle and coarsely ground. This mixture is called dukkah, set this aside.

Whisk remaining 2 tablespoons oil, 2 tablespoons harissa and 1 tablespoon vinegar in a small bowl. remova sweet potatoes from oven and brush with oil mixture. Sprinkle with reserved dukkah.

Dukkah can be made three days ahead. Store airtight at room temperature.