Heat a skillet on medium-high heat and add about 1 Tbsp oil. Begin to sauté onions until fragrant and translucent. Add beef and continue to cook until beef is no longer pink.
Add in taco seasoning and cook according to package instructions. (When using my own seasoning, which I did in this recipe, I always use less water than the packet calls for: ~ ½ cup.) Bring quickly to a boil and then simmer until liquid is almost evaporated.
At this point, add the black beans, corn and green chilies to the beef mixture. Let simmer for ~ 5 minutes to let flavors meld. Liquid should be evaporated.
Spray an 8 x 8 casserole dish with cooking spray. Place three tortillas on the bottom of the dish, cutting them to fit any empty space. Layer with half of the beef mixture. Top with half of the salsa and 1/3 of the cilantro.
Repeat layers. Finish with cheese and the final 1/3 of cilantro.
Bake, uncovered, for 20 minutes or until sides are bubbly and cheese has melted.
Garnish with additional cilantro and serve with sour cream or Greek yogurt.