Mix milk, chili paste and ginger. Boil on the stove, add the shrimp and reduce heat. Boil about 3 minutes. Remove to a plate and set aside. Flour mixed with soy sauce, pour a little of boiled milk, stir and carefully, with constant stirring, add the rest of the soup. Add onions, chopped tomatoes, lime juice and sugar. Allow to boil, then reduce heat and boil for 5 minutes until thickening by occasional stirring. Add chopped mushrooms and boil for another 2 minutes. Shrimp back into the soup and boil until ready. Serve sprinkled with chopped coriander.