Saute the chopped onion in a little olive oil, add the frozen spinach and cook about 10 minutes, stirring often.
Add the rotel tomatoes, cumin, chili powder to the spinach mixture and mix well. Cook about 5 minutes, then add cream cheese and cook until melted on medium low heat. Stir frequently while it melts.
Add black beans to mixture, then add shredded cheese. Mix well then take off heat and put to the side.
To make the Sauce:.
Combine all the ingredients in a sauce pan and mix well. Heat until bubbly.
Spread enough sauce to make a thin layer onto the bottom of a 9 x 13 inch pan. Heat the tortillas in the microwave for about 30 seconds until warm. Spread spoonfuls of the spinach filling onto each tortilla, then wrap it and place seamside down in the pan. When pan is full, pour the remaining sauce over enchiladas, the top with cheese.
Bake at 350 about 30 minutes until sauce is bubbly and cheese is melted.