Place tomatillos, onions, garlic and peppers in a single layer in a oiled baking panand roast in a preheated 400 degree oven until soft and golden brown, 15-20 minutes. Place all of vegetables including juices in a food processor along with cilantro, spinach and stock and process in short bursts until smooth. Add more stock if necessary to make a medium textured sauce. Season to taste with salt and pepper.