Brown the ground beef with the diced onion until the meat is fully cooked; drain the grease and then return the beef and onion back to the frying pan.
Add the drained/rinsed black beans, the salsa and the taco seasoning, stirring to combine.
Simmer until the liquid (from the salsa) has reduced slightly, thickening the mixture. (you can add salt and pepper if you feel it needs it, but I found our salsa to be salty enough for the entire pan of filling)
Can then be used immediately for tacos, burrito filling, or for a "taco bake" (baked under a layer of cornbread batter), OR as a tex-mex pizza topping.
Notes
This makes a lot of burrito filling (more than our family of five can eat in one meal), so I generally use half and freeze the other half (in a freezer bag) for use at a later time.
Recipe by Following In My Shoes at https://followinginmyshoes.com/my-recipes/burrito-pizza-night-black-bean-beef-tex-mex-filling/