Method:
1. Heat a char-grill pan over medium-high heat. Drizzle garlic and onion with oil, place cut-side down on the grill, cook 3-4 minutes until charred.
2. Poor the chicken stock in a saucepan and bring to the boil, add the head of garlic onion, chilli, ginger, coriander root, lime juice, fish sauce, caster sugar, chicken thigh, udon noodles and mushrooms. Simmer for 10 minutes or until the chicken is cooked through.
3. Ladle soup into a bowls, top with fried shallots. Serve.