Chile con Queso

Method

1. Set 1 poblano directly on a gas flame or under a hot grill and turn it every couple of minutes until the skin is blistered and black all over, 6-8 minutes. Drop it into a bowl and cover tightly with a plate or cloth so it steams for 10 minutes.
2. Rub the loosened skin off the chile, pull out the stem, seeds and veins, and cut the flesh into strips about 5 mm wide — the rajas. Keep any juice that collects.
3. Melt 1 tsp butter in a small heavy frying pan or cazuela over medium heat. Add 30 g finely chopped white onion with a pinch of the salt and cook, stirring, for 3 minutes until translucent but not browned.