Roast and peel your poblanos. This is easiest using a BBQ but you can do it with your broiler as well. I would not recommend doing it over a gas burner as it makes one unholy mess.
You want to blast your poblanos with direct heat until the skin blisters and starts to char. This will make it way easier to peel them.
Once they are blistered and a wonderful golden black all over transfer them to a bowl and cover with plastic film wrap. Let them steam in their own heat for about 15 minutes.
Once they are cool enough to handle, remove the skin, stem and seeds. Set aside.
Pre-heat a comal or cast iron frying pan over medium heat.
Slice your onion into 3 one inch thick slices (parallel to the equator of the onion so you see big rings). Peel your garlic cloves.
Place the onions, jalapeño and garlic on the dry, pre-heated pan. You want to cook them until the onions are soft and have a bit of char on both sides. Flip your garlic and jalapeño around. The garlic will cooked quickest, followed by the jalapeño and finally the onion slices.
Remove from heat and allow to cool.
Bring a pot holding enough well-salted water to cover the tomatillos completely to a boil. Simmer the tomatillos for about 4 minutes - they will turn pale when they are ready to come out. Allow to cool. You can also roast the tomatillos under the broiler. That is messy and I find there's enough char flavour coming from the onion and peppers so I usually don't bother. Up to you.
Combine the poblanos, onion, garlic, tomatillos, cilantro, and salt in a blender and puree until smooth. You will likely need to add some or all of the first cup of chicken stock to get it go.