Preheat the oven to 350 degrees F. Spray an 8-by-8-inch glass baking dish with cooking spray.
In a heavy saucepan, melt the butter. Add the onion and cook until translucent, about 4 minutes. Add the poblano rajas (strips) and saute for about 2 minutes. Let cool.
Put the artichoke hearts, cream cheese, mozzarella, sour cream and mayonnaise in a food processor and process until smooth. Add the onion-poblano mixture and pulse a couple times until combined, making sure to leave the mixture a little chunky. Season with salt and pepper.
Transfer the mixture to the prepared baking dish and bake until golden brown and melted, about 15 minutes.
Serve with chicharrones or pita chips.
To char the chiles (or any fresh chile): Put the chiles over a gas flame or underneath the broiler and cook until they are blackened on all sides. Enclose them in a plastic bag and let stand for 10 minutes to steam, which will make them easier to peel.