Take the lemongrass stalk and cut it into 2-inch pieces, then use the back of your knife or rolling pin to gently smash it. This will help release its oils and flavour.
Tear the kaffir lime leaves into smaller pieces to release their essential oils as well. If you can’t find kaffir lime leaves, you can use the lime peel.
Slice the galangal or ginger thinly to allow its flavour to permeate the soup.
Tie all aromatics and black peppercorns in a clean, muslin cloth like a potli/pouch.
In a large pot, heat oil. Add minced garlic and vegetables. Saute them for a minute.
Add the vegetable stock, salt, sugar, soy sauce, and the muslin pouch of aromatics.
Bring the soup to a gentle boil over medium heat. Once it starts to boil, reduce the heat to low and let it simmer for about 15 minutes. This will allow the flavours of the aromatics to meld into the soup base.
While the soup base is simmering, slowly add the coconut milk and diced plain tofu. Stir gently to combine. Let the soup simmer for another 10 minutes.
Add the sliced Thai bird’s eye chillies. If you prefer a milder soup, you can remove the seeds from the chillies before adding them. Let the soup simmer for an additional 5 minutes.
Remove the pot from heat, discard the aromatics potli/pouch and stir in the lime juice, coriander leaves or basil.
Taste and adjust the seasoning as needed, adding more soy sauce, sugar, or lime juice to achieve the perfect balance.
Garnish with freshly chopped cilantro and Thai basil.
Serve the soup with lime wedges and chilli oil on the side.