Let's start by either getting our BBQ or cooktop grill nice and hot. If you don't have either go ahead and use the broiler in your oven.
While things are heating up, peel the Onion leaving it whole, then cut it into thick medallions that can hold up to the BBQ.
If the Tomatillos are small just slice them in half. If they are large create medallions with a similar thickness to the Onions.
Coat the Onions, Tomatillos and Poblano Peppers lightly with Oil and season with Salt. Place them on the grill and sit back and relax for a couple minutes while they start to cook and get some grill marks.
The Tomatillos will probably finish cooking first so remove them when they have some nice grill marks. After that the Onions will come off and finally wait until the Poblano Peppers are Black, and I mean BLACK, all the way around. Make sure to turn the Peppers as each side blackens before taking them off the grill.
The Onions and Tomatillos can go straight into the blender to cool down. The Peppers should either go into a zip lock bag or a bowl covered with seran wrap as they cool. This will help the skins release off the meat and make them easier to peel.
Use the back of a bread knife or a spoon and gently slap the black off the cooled Peppers. Don't worry if you don't get all the black off.
Place everything into the blender and Pulse away until you have something that resembles a semi smooth Salsa.
This can be used right away, placed in the fridge for use in the next few days or canned and stored for the future.
Notes
You can make this Salsa Verde Vegetarian Friendly by substituting Chicken stock for Vegetable Stock.