Remove the husks from the tomatillos, rinse them under water, then add them to the saucepan.
Cut the stem off the jalapeño, split it in half, dispose of the seeds, then chop the halves into half-inch chunks before adding them to the pot.
Peel the outer skin from the half onion then cut it in half so that it is a quarter size of a whole onion before adding it to the saucepan.
Smash the garlic cloves with the side of a chefs knife. Remove their peels then add to the pot.
Add the pork stock (if using) and water to the pot and simmer over medium high heat for 17-18 minutes.
Add the freshly ground black pepper and kosher salt.
Check to confirm the ingredients have softened by poking them with a fork or knife. They should show no resistance when poked. Remove the pan from heat.
Start to blend the ingredients with an immersion stick blender by pulsing it on and off for 3-second intervals 4-5 times. This is a small batch, so you might need to tilt the pot and move the immersion stick blender to a different area of the pot between pulses. You are trying to maintain a slightly chunky texture. (You can also use a food processor)
Taste the sauce to see if it might need a touch more kosher salt. Add the cilantro leaves.
Blend with the stick blender for 2 more 3-second pulses to chop the cilantro into smaller pieces while combining them into the salsa.
The tomatillo salsa verde is ready to serve with tacos or chips.