In a blender, place the tomatillos, 1/3 cup white onion, garlic, cilantro, water, chicken bouillon, salt, and sugar. Blend until smooth.
In a 3-quart saucepan, over medium-high heat, heat 1 tablespoon oil. Add the tomatillo mixture and cook for about 5 minutes or until the sauce slightly thickens.
Meanwhile, in a bowl, mix the sour cream with the milk until well incorporated. Cut the tortillas into 6 triangles each.
In a 12-inch skillet, heat 1/2 cup oil and slowly add the tortilla triangles, about 12 pieces at a time, and cook for 3 to 5 minutes, turning them once, until they become golden brown and crispy.
Place the tortilla triangles on a paper towel and set aside. On a platter, place a layer of tortilla triangles.
Top with the chicken, cheese, and sour cream mix. Repeat for a second layer. Garnish as desired and serve.