Combine tomatillos, garlic, chile and cilantro in a blender jar. Add 1⁄4 cup water and 1 teaspoon salt. Begin blending on low, at first pulsing to get the mixture moving evenly through the blender blades. Blend to a coarse puree. Taste and season with additional salt, if you think necessary. Pour into a serving dish and stir in the avocado.
In a small saucepan, combine the soy, water, mirin and sugar. Simmer over medium heat until just beginning to thicken, 15 to 20 minutes.
In a wok, heat the oil over high heat. When just beginning to smoke, add the duck and stir-fry until browned. Dribble in 2 or 3 tablespoons water, stir-fry a minute longer (to soften any overly crispy edges), then add 1⁄4 cup of the sauce. Stir-fry a minute longer, until the duck is glazed. Scoop onto a serving platter. Serve with warm corn tortillas, the tomatillo salsa and the remaining sweetened soy sauce.