Heat oil in a large pot and add red chili flakes. Saute for a few seconds and then add vegetable stock.
Crush the lemongrass stalk, galangal, and kaffir lime leaves. Put them into the pot and cover it. Cook for 10-15 minutes on medium heat.
Remove the lemongrass, galangal, and kaffir lime leaves from the stock and discard.
Add mushrooms, soy sauce, Thai red chilies, and sugar. Mix and cover the pot. 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 turn off the heat 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 shared in the post above, please follow them to make this recipe.