Heat oil in a Dutch oven on medium. Add onions and sauté for 3 minutes.
Add mushrooms and sauté for 2 minutes. Add garlic and ginger; sauté for 30 seconds.
Add red curry paste and stir to coat.
Add lemongrass, chilies, broth, coconut milk, lime juice, fish sauce, coconut sugar and lime zest. Stir well, increase the heat to medium-high and bring the mixture to a soft boil.
Add shrimp and cook until they just turn pink, about 2 minutes.
About 30 seconds before time is up, add tomatoes, stir and cook for the remaining time.
Remove lemongrass stalks and chilies; discard them.
Increase the heat to high and bring to a boil.
Add cornstarch mixture and stir until the mixture thickens, about 45 seconds.
Spoon soup into warm bowls, garnish with chopped cilantro and serve with lime wedges.