This dressing is a stretch for St. Patrick's Day (Mexico is a far cry from Ireland!) - but it does fit into the GREEN category! (About now you may be tiring of Irish fare...)
In my neighborhood we used to have mini cooking demonstrations where we share our favorite recipes with each other. A few years ago, Carol - a great cook - hosted a demo of Pork Barbacoa with this lightly seasoned creamy dressing.
A tomatillo is a fruit that belongs to the same "nightshade" family as the tomato. Also called a "Mexican green tomato," it does look like a small green tomato with a papery husk in place of skin. But it has a tart flavor all its own.