Teriyaki Chicken Noodle Soup

Name: Teriyaki Chicken Noodle Soup
Ingredients: 1 cup lower-sodium soy sauce; 1/4 cup light brown sugar; 1/4 cup mirin; 1 tablespoon grated fresh ginger (from one 5-inch piece); 2 teaspoons grated garlic (from 4 medium cloves); 1 1/2 pounds boneless, skinless chicken thighs; 2 ( 7.1 - ounce ) packages instant udon noodles; 4 medium scallions; 3 tablespoons grapeseed oil , divided; 6 cups low-sodium chicken broth; 3 bunches baby bok choy , separated into individual leaves; 1 cup sliced baby bella or crimini mushrooms