Step 1

Preparation Preheat oven to 400°F. Whisk 2/3 cup oil, 2/3 cup harissa, and 1 Tbsp. vinegar in a large bowl to combine. Slice sweet potatoes crosswise on a mandoline 1/8" thick. Add to bowl with harissa mixture and toss to coat; season with salt.

Step 2

Arrange sweet potatoes so they are standing upright in concentric circles in a 2-qt. baking dish, packing tightly, and roast, brushing any accumulated harissa oil in dish onto sweet potatoes every 10–15 minutes, until soft and starting to brown on top, about 1 hour.

Step 3

Meanwhile, toss pistachios, sesame seeds, and fennel seeds on a small rimmed baking sheet. Toast alongside sweet potatoes until golden brown, about 4 minutes. Let cool, then transfer to spice mill or mortar and pestle and coarsely grind. Set dukkah aside.

Step 4

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

Step 5

Do Ahead Dukkah can be made 3 days ahead. Store airtight at room temperature.