For plum vinaigrette
1. Preheat oven to 400°F. Prepare plum vinaigrette by adding all vinaigrette ingredients in a mason jar and shake.
2. Slice sweet potatoes crosswise with mandolin 1/8" thick. Combine with plum vinaigrette in a bowl. Season with salt and toss.
3. Arrange sweet potatoes so they are standing upright in concentric circles in a 2-qt. baking dish, packing tightly, and roast. Brush any extra plum vinaigrette from dish onto sweet potatoes every 10–15 minutes, until soft and starting to brown on top, about 1 hour.
4. 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.
5. Remove sweet potatoes from oven and brush with vinaigrette. Sprinkle with reserved dukkah.
Delicious, farm-fresh harvests are closer than you think.