Add all of the ingredients (other than the meat) to your slow-cooker. No need to stir. If you're not adding meat, turn on high 4 hours. Serve as a taco filling, or mix with cooked sweet corn kernels as a thick soup.
If using chicken breasts, nestle those on top and turn it on to high for 5-6 hours. Remove the chicken and shred it, and either mix it back into the tomatillo mixture or serve separately, reducing the sauce for a while first.
If using pork, trim off any excess fat ribbons first and then salt and pepper and sear it over high heat in some oil, 4 minutes per side. Rest it on top of the other ingredients and turn the cooker on to high for 5-6 hours. Pork is done when it shreds easily. Same as with the chicken, remove it to shred and serve separately with the sauce on top (reduce the sauce for a few minutes first), or mix the shredded meat back into the pot and serve all together.