Heat oil in skillet. Add chopped onion and salt. Saute until softened. Add garlic, chili powder, and bell pepper. Mix together and saute another 5 minutes. Add chicken, green chilies, and spinach. Stir together, cover, and simmer 5-10 minutes, until chicken is cooked through and all ingredients are mixed well.
Heat oven to 350F and spray a two-quart casserole dish with cooking spray. Pour 1/2 cup salsa in the bottom of the dish. Put a layer of tortilla chips on the salsa, then layer half the chicken mixture and half the cheese. Repeat the layers, but do not add the top layer of cheese at this point.
Cover and bake for 20 minutes, until heated through. Top with remaining cheese and bake uncovered for 15 minutes, until cheese is melted and starting to brown. Let stand at room temperature for 5 minutes before serving.