Uncle Don's Double Mustard Greens & Roasted Yam Soup

1 large garnet yam (about 1 pound) peeled and cut into 1/2-inch chunks

2 tablespoons extra virgin olive oil

Coarse sea salt

6 cups vegetable stock

1 large bunch mustard greens (about 1 pound), tough stems removed, chopped into bite-size piece, washed, and drained

1 teaspoon mustard seeds

3 cloves garlic, minced

2 teaspoons apple cider vinegar

white pepper