Chicken and soba noodle soup

Name: Chicken and soba noodle soup
Ingredients: To serve:; 400 grams Japanese soba noodles; 250 grams mushrooms, chopped lengthways; 2 stalks of spring onions (only the green parts), chopped; For the chicken stock:; 1 1/2 pounds chicken pieces, bone in; 1 leek, chopped into 3” pieces; 3 carrots, chopped into thick rounds; 2 onions, quartered; 1 whole head of garlic, halved crosswise; 1 bunch coriander, roughly chopped (avoid the leaves); 15 black peppercorns, bruised; 1 sprig thyme (optional); Salt, to taste