A classic, dressed up with Thai flavors that might just turn this soup into a regular around your home.
In a large pot on medium low heat, add the butter and olive oil. While the butter melts, smash the lemon grass with the side of your knife to open it up, and make a slit through the middle of each Thai chili, while leaving the stem uncut, to open it up as well.
Add the lemon grass, chilis, and smashed garlic cloves to the oil and sweat on medium low heat for 3-5 minutes.
Remove from heat and add the crushed tomatoes. Immediately cover with a lid to avoid spatter. Once the spatter stops, remove the lid and stir. Put back on medium low heat and add the fish sauce and bone broth. Stir and cover, allowing to simmer for 15-20 minutes.
Remove the lemon grass stems and chilis. Blend the soup together until smooth (either with an immersion blender or in a regular blender. if using a regular blender work in small batches and put everything back in the pot when done).
Add the coconut milk and stir. Bring to a simmer, add the fresh basil, stir and turn off the heat. Cover and let sit for 5 minutes. Stir before serving. It's encouraged for each person to squeeze a little lime into their bowl before eating.