4 sweet potatoes (about six ounces each), scrubbed 1 clove garlic, chopped 1 (1-inch) knob of fresh ginger, peeled and grated A few pinches of sugar or 2 teaspoons mirin 1 heaping tablespoon white miso 1 tablespoon unseasoned rice wine vinegar 1 tablespoon light sesame oil or other neutral oil, plus more for the pan 1 tablespoon toasted sesame oil 2 teaspoons toasted black sesame seeds, for garnish