Heat oil in a pot and add chili flakes. Saute for a few seconds and then add vegetable stock.
Crush the lemongrass stalk, galangal, and kaffir lime leaves. Add them to the pot and cover it. Cook for 10-15 minutes on medium flame.
Remove the lemongrass, galangal, and kaffir lime leaves from the stock and discard.
Add mushrooms, soy sauce, and sugar. Mix and cover the pan. Cook for another 10 minutes.
Add cherry tomatoes and cook for just 1-2 minutes. Don't overcook the tomatoes.
Finally, add lime juice and chopped cilantro. Serve hot.
Use any variety of mushrooms that's available to you.
You don't necessarily need to remove kaffir lime leaves, lemon stalk, and galangal from the broth. If you don't mind them in the soup, you can keep them. However, in that case, just make sure to make small pieces of lemongrass and galangal before adding to the soup.
Chili flakes can burn very quickly so make sure to switch off the gas before adding them and add the stock within a few seconds of adding chili flakes.
Don't overcook the tomatoes in Tom Yum soup. 1-2 minutes of cooking is enough in this recipe. Don't allow the tomato skin to fall off.
More detailed tips for this recipe are given within the post above.