Soak mushrooms in 3 cups (750 mL) warm water until softened, about 30 minutes. Drain, reserving liquid; strain liquid through fine sieve. Cut off mushroom stems; set aside. Cut mushroom caps into thin strips.

Cut lemongrass into 2-inch (5 cm) lengths, discarding woody ends. Crush with side of knife to split; set aside.

Separate leaves and stems from coriander; set aside.

In saucepan, heat oil over medium heat; fry lemongrass, coriander stems, shallots, ginger, half of the hot peppers, garlic, peppercorns and cloves until fragrant, about 3 minutes. Stir in stock, fish sauce, mushroom stems and reserved liquid; bring to boil. Cover and reduce heat; simmer for 20 minutes.

Strain and return to saucepan. Add mushroom strips and simmer for 5 minutes. (Make-ahead: Refrigerate for up to 1 day.) Mix cornstarch with 2 tbsp (30 mL) water until smooth. Stir coconut milk into soup; bring to simmer. Stir in cornstarch mixture; simmer for 1 minute, stirring constantly. Stir in lime juice.

Garnish with coriander leaves and remaining hot peppers.

Makes 6 servings.