Trim off the top third and very bottom of the lemongrass stalks. Using the side of a knife or a meat mallet, press down on the stalks to crush the lemongrass. Chop each stalk into 2-4 pieces and set aside.
Melt the coconut oil in a large pot over medium heat. Add the lemongrass, onion, ginger and chili paste and cook for 3-4 minutes, until fragrant.
Add seafood stock, water, fish sauce, and lime zest. Reduce heat to low, and simmer uncovered for 20 minutes.
Add coconut milk, shrimp, mushrooms and tomato. Cook for 5 minutes, until shrimp are cooked through. Stir in lime juice and season with salt and pepper to taste.
Remove the lemongrass pieces or ask your guests to set them aside as they eat the soup. Serve hot and garnish with cilantro and lime wedges.
Store leftovers in the refrigerator for up to 4 days or freeze for up to 4 months.