In a bowl, whisk together the boiling water and the brown sugar, stirring until the sugar dissolves. Add the fish sauce, Sriracha, tamari, and garlic and stir to combine. Set aside about 3 tablespoons marinade to drizzle over the sandwiches later. Transfer the rest of the marinade to a shallow baking dish and let cool to room temperature, 10 to 15 minutes. Place the salmon slices in the marinade, turning to coat both sides. Cover and refrigerate for about 1 hour, flipping the salmon halfway through.