Brown the ground beef in a large pan. Season with salt, pepper, and red pepper flakes to taste.
Add frozen spinach and diced tomatoes to pan and cook just until the spinach is incorporated, about 1-2 minutes.
Combine the beef and vegetable mixture with the black beans and 1 cup of the salsa (reserve ¼-1/2 cup salsa), season to taste with more salt, pepper, and red pepper flakes if desired.
Coat the bottom of a 9 x 13-inch casserole dish with a little salsa, then start layering: corn tortillas, a little salsa, beef/bean/vegetable mixture, cheese. You should get 4 layers.
Top the final layer with cheese. Bake, uncovered, for 20 minutes on 350F.
Notes
I've also replaced half of the salsa with 1.5 cups of canned diced tomatoes. It works in a pinch, or if you want to reduce the sodium. I recommend a basic salsa here - I use Trader Joe's red Salsa Autentica for $1.79. Cheese recommendations: cheddar, queso fresco, monterey jack, pepperjack, or some combination of those! (My fave is sharp white cheddar combined with queso fresco.)