Over an open flame or 4 inches below a preheated broiler, roast the poblanos, turning them regularly until evenly blistered and blackened all over, about 5 minutes for an open flame, about 10 minutes for the broiler. Place in a bowl, cover with a kitchen towel and let cool.
Rub off the blackened skin, then pull out the stem and seed pod; briefly rinse to remove any bits of skin and stray seeds. Chop into ¼-inch pieces and scoop into a medium bowl.
Place the onion into a strainer, rinse under cold water, shake off the excess, and transfer to the bowl along with the mushroom (or smoked salmon), oregano, and lime juice. Taste and season with salt if using mushrooms, usually about ½ teaspoon (if using smoked salmon you won’t need salt), if needed. Cover and refrigerate.