To make the sauce, husk the tomatillos and rinse under warm water to remove stickiness and dirt. Broil on foil baking sheet 1 to 2 inches from heat, turning once. till soft and slightly charred, about 7 - 9 mintues.
Heat oil in large skillet over med. heat. Add onions and saute about 5 minutes.
Transfer tomatillos to a blender and add the sauteed onions, cilantro, green chilies, cumin, sugar and salt; puree till smooth.
Return tomatillo sauce to skillet and cook over med. heat till thickened about 5 minutes, stirring frequently to prevent sticking. Add the broth and cream and continue cooking till the sauce is slightly thickened and not watery, about 5 - 8 minute Add more salt if desired.
To make enchiladas, brush both sides with oil and lay on cookie sheet in stacks of two. Heat in 375 over just until soft and pliable, about 3 minutes.
In large bowl, combine the chicken, half of the cheddar. half of the monterey jack, the chopped onion and the salt. Add 1/2 cup of the sauce to the chicken mixture and toss to combine.
Spread a third of the remaining sauce in the bottom of a 9 x 13 inch baking dish. Place 1/4 cup of the chicken mixture in the center of a tortilla. Roll up the tortilla and place seam down in the dish. Repeat with the other tortillas. Pour the rest of the sauce over the enchiladas. Sprinkle with the remaining jack and cheddar cheese on top of the casserole.
Bake the casserole until the enchiladas are heated through and the cheese is bubbly and starting to brown, about 20 - 30 minutes.