Tom Yum Paste: Combine all the ingredients up to the cilantro in a food processor. Pulse until a thick paste forms and all the ingredients are broken down.
Heat the tom yum paste in a big soup pot over medium high heat. Cook for about 30 seconds.
Add in the chicken broth, water, and onion. Bring the soup to a boil.
Once boiling, add in the mushrooms, tilapia, mussels, tomatoes, and shrimp. Let it simmer for 15 minutes until all the seafood is cooked.