Mix the baking soda in a large bowl or very clean sink with enough water to submerge all the mussels, and cover them by a few inches. Gently plunk molluscs in there while you get on with making the broth. You don’t want to leave them in here any longer than 10 minutes, give or take.
In a large Dutch oven or braising pan, combine the oil, garlic, ginger, lemongrass, one chili and the cilantro stems. Heat over medium heat until it begins to sizzle gently and become fragrant.
Add the coconut milk and bring to a gentle boil. Reduce the heat and simmer for 10 minutes and no more. Strain out the broth through a cheesecloth-lined sieve, into a liquid measuring cup and discard all the solids.
Wipe the pan clean and add the broth back in. Bring to a boil. Drain and rinse the mussels under cold water, then add them to the pot and cover. Drop the heat to the very lowest of low and simmer for 5 -7 minutes, or until the mussels are almost completely open. Give the pan a gentle shake every now and then.
While the mussels steam, toss the tomatoes, remaining Thai chili, green onion, cilantro leaves and lime juice together.
Transfer the mussels to two very large, wide serving bowls and top with the tomato salad. Serve with the toasted focaccia.