You’ll find tomatillos in the produce section at most grocery stores. Slideshow: More Mexican Recipes
In a blender, puree 1 pound of the tomatillos, chile, scallions, cilantro, lime juice, 1/2 teaspoon salt and 1/4 teaspoon pepper. Season with salt and pepper to taste.
In a large heavy skillet, heat the oil over medium high heat until hot, then stir in the on-ion, garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook, stirring occasionally, until golden, about 6 minutes. Stir in the chicken, the remaining tomatillos and sugar, then cook, stirring until the chicken is cooked and the tomatillos are softened, about 6 minutes.
Divide the filling between the tortillas. Roll each tortilla into a cylinder and place, side-by-side in a baking dish. Pour the reserved salsa verde over the tortillas.
Bake the enchiladas until the filling is hot, about 10 minutes. Serve garnished with cilantro sprigs and tomatillo slices.