Bring first 4 ingredients to boil in large skillet. Reduce heat to medium-low, cover, and simmer until tomatillos are soft, about 15 minutes. Transfer mixture to blender; add 2 cups onions, green chilies, and spinach; puree. Season sauce with salt and pepper. Return sauce to skillet. Preheat oven to 350°F. Lightly oil 13x9x2-inch glass baking dish. Dip 1 tortilla into sauce. Place tortilla on plate. Top with 2 tablespoons cheese, 1 tablespoon remaining onions, and 1 tablespoon sauce. Roll up tortilla. Place seam side down in prepared dish. Repeat with remaining tortillas, sauce, cheese, and onions. Pour remaining sauce over enchiladas. Sprinkle with remaining cheese. Bake until cheese melts, about 15 minutes.