1 plum tomato—halved, seeded and cut into 1/4-inch dice
1 tablespoon chopped cilantro
1 tablespoon chopped dill
Heat a medium cast-iron skillet. Add the shallots, garlic and chiles and cook over moderate heat for 3 minutes, stirring occasionally. Add the scallion pieces and cook, stirring occasionally, until everything is charred in spots, about 3 minutes longer. Transfer the contents of the skillet to a large mortar and pound to a coarse paste.
In a medium skillet, bring the water to a boil. Add the mushrooms and boil over high heat until all of the water has evaporated, about 4 minutes. Add the fish sauce and cook, stirring, for 1 minute. Transfer the mushrooms to the mortar and pound until coarsely mashed with the shallots and chiles. Transfer the mushroom-chile sauce to a bowl and stir in the tomato, cilantro, dill and sliced scallion. Season with salt and serve.