Preheat oven to 400 degrees.
Boil Manicotti in salted water for 8 minutes; drain, reserving 1/2 cup of pasta water.
In a large skillet, heat oil over medium heat. SautÃ© onion until translucent. Add garlic & sautÃ© 1 minute more. Remove pan from heat & mix in chicken, tomatoes & cilantro. Set aside.
For Tomatillo Sauce, place tomatillos, onion, jalapeno & garlic on a baking sheet. Toss with oil & season with salt & pepper. Roast until tomatillos turn golden around the edges & are soft, about 40-45 minutes. Remove from oven & blend ingredients together until smooth. Use reserved pasta water to thin sauce, if necessary. Season the sauce with salt & pepper to taste.
For White Cheese Sauce, in a medium sauce pan, melt butter over medium-high heat. Add flour & cook 2 minutes. Whisk in heavy cream & bring to a simmer. Add cheese & nutmeg & stir until melted. Season the sauce, with salt, to taste.
Reduce oven temperature to 350 degrees. Stuff manicotti shells with chicken mixture & assemble in a 9x13" baking dish. Pour cheese sauce over the top. Bake 30 minutes until cheese is bubbling.
Drizzle tomatillo sauce on maniladas before serving.