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1
Preheat oven to 375u00b0F. Toss broccoli with 2 tablespoons olive oil and spread in an even layer on 2 rimmed baking sheets. Transfer to oven and roast until broccoli is tender and browned, about 25 minutes.
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2
In a Dutch oven, heat 2 tablespoons olive oil over medium-high heat until shimmering. Add onion and garlic and cook, stirring, until softened, about 3 minutes. Stir in 1/2 teaspoon coriander seed and red pepper flakes and cook until fragrant, about 1 minute.
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3
Add roasted broccoli, buttermilk, and just enough stock to barely cover vegetables. Bring to a simmer, then remove from heat. Working in batches if necessary, transfer vegetables and liquid to a blender. Starting on the lowest speed and gradually increasing to the highest speed, blend broccoli until a smooth puree forms. Drizzle in 4 tablespoons total olive oil while blender is running and return to a clean pot. Add as much of the remaining vegetable stock as necessary to thin to a creamy soup consistency. Season with salt.
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4
Meanwhile, lower oven to 350u00b0F. In a mixing bowl, toss pepitas with 2 tablespoons olive oil, remaining 1/2 teaspoon ground coriander, cumin, mustard seed, and turmeric. Season with salt. Transfer to a parchment-lined baking sheet and roast until fragrant, about 10 minutes. Let cool.
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5
Scrape toasted spiced pepitas into a mixing bowl and stir in parsley, lemon zest, and remaining 2 tablespoons olive oil.
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6
Warm the soup, then spoon it into bowls. Top with spiced pepitas and some of their oil. Serve right away.
