Saute onion in a little butter & OO in skillet over medium heat.
When onion is transparent and starting to brown, toss spinach on top of it and let it steam for a minute, then stir and cook 1-2 more minutes until wilted and cooked down.
Drain black beans and rinse well. Combine taco meat, corn, beans, spinach & onion.
Lay out rows of tortillas and smear a little sour cream down the center of each [1 T or so] divide filling among tortillas and top with a little cheese.
In a prepared casserole dish, pour a can of green enchilada sauce.
Roll up tortillas tightly and place them seam side down in pan, squishing tightly together.
Spread remaining sour cream over tortillas, cover with a can of red enchilada sauce and top with cheese.
Cover and bake at 350 for 30 minutes, then uncover and cook until hot and bubbly. This makes 1 big [serves 4-6] and 1 small [serves 2-4] casseroles, or 1 extra large pan [serving 10] Can be made a day ahead.
Serve with a green salad and garnish with tortilla chips.