1. Preheat oven to 400 F. Line a large baking sheet with parchment or foil. 2. Heat a large, deep frying pan over medium heat. Add oil and then the onion. Cook for 3 to 5 minutes or until slightly softened. Add garlic and cook for another minute. Stir in chili powder. Add in tomatoes and tomato paste. Simmer, stirring and crushing tomatoes with a wooden spoon, until it starts to thicken, about 5 minutes. Add spinach and cook until thawed. Add black beans, chilies, chicken, corn and salt. Stir well and cook until heated through and bubbling, about 5 minutes. Remove from heat. 3. Place one tortilla on prepared baking sheet. Scoop about one-sixth of bean mixture onto tortilla and spread out to the edges. Sprinkle with about 1/4 cup of cheese. Top with another tortilla. Repeat layers, ending with all the remaining bean mixture and sprinkling with all of the remaining cheese. 4. Bake for 20 minutes or until hot and bubbly. Broil up to 5 minutes, if desired, for extra colour and bubbliness on top. Let stand 5 minutes before serving. Serve with sour cream, lime wedges and cilantro if desired.