First, turn on your oven to 425°F (218°C) with a sheet pan inside. Let it come to temp and get everything nice and hot for at least 30 minutes.
While the oven and pan are heating up, clean your tomatillos by removing their husks and washing off the stickiness. Slice the jalapeno(s) or serrano(s) down the center and remove the seeds depending on how spicy you'd like the resulting salsa. If making the poblano version, slice the poblano peppers in half and remove their seeds as well. Put the tomatillos and peppers into a medium-large bowl and drizzle with oil. Toss everything to coat. Set aside until ready to cook in the oven.
Next, take out your blender and set it aside on the counter with the lid off. If you want to toast your spices (optional), now's the time. Bring a stainless-steel pan to medium heat on your stove top and add the black peppercorns, coriander, and cumin. Move the spices around the pot by shifting it back and forth every 30 seconds or so. Once you can smell the spices, take them off the burner and dump them into your open blender. (See notes)
Add in salt and the middle ⅓ of your cilantro bunch as well as the fresh garlic cloves, rough chopped onion, and lime juice.
Your oven and pan should be properly pre-heated by now. Carefully pull the pan out of the oven and add the prepped peppers and tomatillos onto it. Place it back into the oven and let them roast for 10-15 minutes. Then, turn your oven to broil and let them char for another 2-5 minutes, until the skin on the peppers starts to blister and turn black.
Take the roasted ingredients out of the oven and use tongs to add the tomatillos to the blender. Don't worry if your tomatillos have broken open, we will get to that in a bit. If you want to peel your peppers (also optional, see FAQS for an explanation) place the peppers in a medium bowl and cover it with plastic wrap for 5 minutes.
While the peppers steam, deglaze the roasting pan. Add ¼-⅓ c (60-80ml) water to a small sauce pot and bring it to a boil. Then, pour the water over the browned sections or fond (e.g. tomatillo guts) on the pan. Make sure the pan is cool enough to handle before the next step or use a towel/mitt. Using a spoon, bench scraper, or spatula with a flat edge dissolve and scrape up the fond until the water has turned brown and collected all of the flavor. Pour the pan drippings into the blender.
Next, remove the plastic wrap over the pepper bowl and peel off as much of the pepper skin as possible. Add the peeled peppers to the blender and blend everything together to your desired consistency. If you want to make the third version of salsa verde with avocado, this is where they get added. Slice them in half, remove the pit, and scoop the avocado meat out with a spoon into the blender. Stop when the salsa becomes uniform and creamy. Enjoy!