Beef and mushroom noodle soup

Name: Beef and mushroom noodle soup
Ingredients: 250 g very thinly sliced beef rump (make sure it is sliced across the grain); 8 whole dried shitake mushrooms; 6 cups beef stock; ¼ cup light soy sauce; 4 spring onions, coarsely chopped, plus extra thinly sliced to serve; 5cm piece ginger, peeled and coarsely chopped, plus extra finely grated to serve; 1 head of garlic, halved; 4 small bok choy, cut into 3cm pieces; 2 tablespoons mirin (for alternative, see note); 300 g dried udon noodles; Sesame seeds and a drizzle of sesame oil, to serve