Chicken Udon

Name: Chicken Udon
Ingredients: 5 1/2 Cups low-sodium chicken stock; 2 Tablespoons soy sauce or tamari; 2 Tablespoons mirin; 8 Ounces boneless chicken breast, cut into slivers; 4 medium-sized shiitake mushrooms, sliced; One 9-ounce package dried or frozen udon noodles; 4 handfuls of baby or regular spinach, stemmed; 1 Tablespoon hot sesame oil; 2 scallions, julienned into 2-inch pieces, for garnish