Heat the sesame oil over medium low heat, then throw in the shallots. Allow them to soften for a minute or two, then gently stab the slices with chopsticks or a spatula to break them up. Throw in the shiitake caps, stir, and allow to soften for two minutes. Sprinkle the brown sugar over the top, then stir again to combine.
Increase the heat to medium, then throw in the chopped scallions and broth. Stir, then allow to come to a small simmer. Allow it to simmer for about two minutes, then remove it from the heat and stir in the coconut milk, lime juice and tomatoes.
Serve immediately with a few tofu cubes thrown in if you like. Garnish with hot peppers or sriracha and cilantro, if desired.