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1
Preheat oven to 300u00b0F. Toss sweet potato with 4 tablespoons vegetable oil and salt. Spread sweet potato on 2 rimmed baking sheets and bake for 1 hour, rotating pans front to back and top to bottom halfway through cooking. Increase oven temperature to 375u00b0F and cook until sweet potatoes are tender and lightly browned around the edges, about 25 minutes.
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2
In a large pot, heat remaining 2 tablespoons vegetable oil over medium-high heat until shimmering. Add carrot, onion, garlic, sage (if using), and a generous pinch of salt and cook, stirring, until vegetables are just tender.
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3
Add sweet potatoes and stock and bring to a simmer. Cook for 10 minutes until vegetables are very tender. Discard sage, if using. Add orange juice. Using an immersion blender or standing blender, blend soup until smooth. Season with salt.
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4
If desired, pass soup through a fine mesh strainer using a rubber spatula to plunge and scrape it through (it may not seem like it at first, but almost all of the soup will pass through). If necessary, thin soup to desired consistency with extra stock or water.
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5
In a mixing bowl, stir together pistachios, orange zest, scallions, mint, and cayenne pepper. Fold in olive oil and season with salt.
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6
Serve soup, spooning pistachio salsa on top.
