In a bowl, combine the sliced chicken breast with soy sauce and cornstarch. Mix well and set aside to marinate for 15-20 minutes.

Heat the cooking oil in a large pot over medium heat. Add the sliced onion, minced garlic, and bruised lemongrass stalk. Sauté until the onions are translucent and fragrant. Stir in the diced tomatoes and cook until they start to soften. Pour in the chicken broth and water. Bring the mixture to a gentle simmer.

Dissolve the tamarind paste in a little warm water to create a tamarind liquid. Add it to the simmering soup along with fish sauce and sugar. Stir well to combine. Allow the soup to simmer for about 10 minutes to let the flavors meld together.

Once the soup base is well-flavored, add the marinated chicken slices into the pot. Stir gently to distribute the chicken evenly. Let the soup simmer for an additional 10-15 minutes or until the chicken is cooked through and tender.

Remove the pot from the heat. Taste the soup and adjust the seasoning with salt if needed. Ladle the Cambodian Sour Chicken Soup into serving bowls. Garnish each bowl with chopped cilantro, sliced green onions, and red chili peppers. Serve hot with lime wedges on the side.