For the maniladas:
1) Preheat oven to 200 degrees C/Gas 6.
2) Boil manicotti in salted water for 8 minutes. Drain, reserving 120ml of pasta water and set pasta aside.
3) In a large frying pan, heat oil over medium heat. Saute onion until translucent, about 3 minutes. Add garlic and saute 1 minute more. Remove pan from heat and mix in chicken, tomatoes, coriander and sour cream. Set aside.
4) To assemble, reduce the oven temperature to 180 degrees C/Gas 4. Stuff manicotti tubes with chicken mixture and assemble in a 23 by 30cm baking dish. Pour cheese sauce over the top. Bake 30 minutes until cheese is bubbling.
5) Drizzle tomatillo sauce on Maniladas before serving.
For the tomatillo sauce:
1) Place tomatillos, onion, jalapeno and garlic in a baking dish. Toss with oil and season with salt and pepper. Roast until tomatillos turn golden around the edges and are soft, about 45 minutes.
2) Remove from oven and blend ingredients together until smooth. Use reserved pasta water to thin sauce, if necessary. Season with salt and pepper, to taste.
For the cheese sauce:
1) In a medium saucepan, melt butter over medium-high heat. Add flour and cook 2 minutes. Whisk in double cream and bring to a simmer. Add cheese and nutmeg and stir until melted. Season sauce with salt, to taste.