1 large sweet potato (about 1 pound) peeled and cut into 1/2-inch chunks2 tablespoons extra virgin olive oil Coarse sea salt 6 cups vegetable stock 1 large bunch mustard greens, tough stems removed, chopped into bite-size pieces 1 teaspoon mustard seeds 3 cloves garlic, minced 2 teaspoons apple cider vinegar White pepper
Preheat the oven to 425°F. In a large bowl, toss the sweet potatoes with 1 tablespoon of the oil and 1/4 teaspoon of the salt. Transfer the sweet potatoes to a parchment-lined baking dish and roast for 1 hour, stirring every 15 minutes, until they are starting to crisp on the edges.
While the sweet potatoes are roasting, bring the vegetable stock to a boil in a medium saucepan over high heat. Add the mustard greens and cook, uncovered for 4 to 6 minutes, until softened and the sulfur has been released from them. Remove the greens from the heat, drain the stock into a bowl, and set it aside.
Over medium heat in the saucepan just used, warm 1 tablespoon of the olive oil. Add the mustard seeds and cook, stirring occasionally, until they start to pop, 2 to 3 minutes. Next add the garlic and sauté until fragrant, about 1 1/2 minutes. Then add the greens and 1/4 teaspoon salt. Sauté the greens, stirring occasionally, until most of the liquid has evaporated, about 3 minutes. Add the vegetable stock back to the saucepan and set aside.
When the sweet potatoes are done, transfer them to the saucepan. Bring to a boil, then lower to medium and simmer for about 25 minutes until the sweet potatoes are fork tender. Add the apple cider vinegar and white pepper and additional salt to taste.