Heat canola oil over medium high heat. Cook ginger, garlic, cilantro and lemon grass, until fragrant about 4 minutes. Stir in tom yum paste and cook for another minute. Add in chicken broth and bring to a boil, turn heat down and let simmer for 20 minutes. Strain broth and return to pot.
Add in mushrooms, cherry tomatoes, fish sauce and prawns and simmer for another ten minutes. Turn off heat and add in tofu.
To serve distribute soup evenly between four bowls, and garnish with remaining cilantro and lime wedges.