Simmer stock with aromatics: Get out a large pot and pour the 2 quarts of chicken stock in it. Place this over medium-high heat and toss in any of the optional aromatics that you plan on using. Once it comes to a simmer cover the pot and let it all simmer for 20 minutes.
Add the chicken, mushrooms, and onion: After 20 minutes toss in the chicken, mushrooms, and onion. Cover the pot again and let everything simmer for 10 minutes, or until the chicken is cooked.
Add the tomato along with the fish sauce, salt, sugar, red curry paste, and lime juice. Stir everything together and let this simmer for 2 more minutes.
Taste and adjust if needed: Taste the tom yum broth. If it is too salty or spicy then add a little more sugar to balance it out. If it’s to sour then add some more salt to balance it out.
Serve and garnish with cilantro