Using the back of a chef's knife, bash the lemongrass which will help release its flavor. Slice it in half lengthwise and chop it into 1-inch pieces.
In a large pot, combine the broth, lemongrass, ginger, coconut aminos, maple syrup, lime zest and juice, and simmer for about 15 minutes. Strain the broth over a medium bowl, discard the lemongrass and ginger pieces, and set the broth aside.
Return the pot to the stove and heat the coconut oil over medium heat. Add the onion and cook until translucent and soft, about 5 minutes. Stir in the mushrooms and a small pinch of salt. Stir and cook until the mushrooms are soft, about 15 minutes. Stir in the curry paste until well incorporated, then stir in the tomatoes. Add the reserved broth and simmer for 5 minutes, then add the and bok choy. Simmer until the bok choy is tender but still vibrant green, 5 to 7 minutes.
Taste and adjust seasonings, adding more coconut aminos or lime juice if desired. If you would like more spice, add more red curry. If it's too tangy, salty and/or spicy, add the coconut milk. Serve with a scoop of brown rice. Top with baked tofu, if using. Enjoy!