Mix the chicken, garlic, chopped coriander and pepper together. Spoon 1 teaspoon of the mixture into the middle of each wonton sheet. Brush the edge with water and gather up, squeezing the corners together to make a little purse. Lay on a tray and repeat until you have used up all the filling and sheets.
Cook the wonton purses in boiling water for about 2–3 minutes or until they float up. Lift the purses out with a slotted spoon and drop into a bowl of cold water.
Put the coconut milk, stock, lemon grass, galangal, coriander and shallots in a saucepan and bring to the boil. Add the mushrooms and chillies and cook for 3–4 minutes. Add the fish sauce and lime juice. Taste and adjust the seasoning.
Drain the cooked wontons and spoon into the stock saucepan. Add the cherry tomatoes in the last few seconds, taking care not to let them lose their shape.
Place a handful of spinach leaves into each serving bowl. Divide and spoon the hot and sour soup with wontons into each serving bowl.