Arctic Char with Chinese Broccoli and Sweet Potato Purée

Name: Arctic Char with Chinese Broccoli and Sweet Potato Purée
Ingredients: 3 red-skinned sweet potatoes (yams; about 2 1/2 pounds); 1 teaspoon (or more) hot prepared Chinese mustard; 1 cup balsamic vinegar; 1 1/2 teaspoons soy sauce; 1 pound Chinese broccoli or broccoli rabe (rapini),* large outer leaves removed, cut crosswise on diagonal into 1/4-inch-thick slices; 2 slices bacon, cut into 1-inch pieces; 2 teaspoons yellow mustard seeds; 4 5- to 6-ounce arctic char fillets; 2 tablespoons vegetable oil, divided