Place pepper over the flame of a gas burner on medium heat. Turn frequently until the skin is thoroughly charred on all sides. If you don’t have a gas burner, roast pepper under the broiler.
Remove charred pepper to a bowl and cover with plastic wrap. Allow to cool enough to handle. Wipe off charred skin, stem and seed the pepper, then thinly slice. Reserve.
For the guacamole, cut the avocado in half and remove the pit. Scoop out the flesh and transfer to a bowl with the lime juice and 1/4 of the minced yellow onion. Lightly smash the avocado with a fork or masher to combine. Season with salt and pepper, to taste. Cover with a piece of plastic wrap, pressing it down directly on top of the guacamole and reserve.