Preheat oven to 350. De-bone chicken, place in medium mixing bowl, set aside. In a small saute pan, heat oil and add onions, cook until almost tender, add garlic and saute a few more minutes, add to chicken. Add cilantro(optional)tomatoes, sour cream, mix well. Salt & pepper to taste.
Boil manicotti shells until almost done, (salt water well)drain and let cool. Stuff with chicken mix and place in baking dish (9x13)
Cheese Sauce: In a sauce pan add the butter and flour, stir until combined well, slowly whisk in cream and cook over low-medium heat for 5-8 minutes letting it thicken some, add nutmeg and cheese, stir to melt cheese. Pour over manicotti shells, place in preheated over and bake for 30 minutes. Serve with garlic bread and green salad