1 very small onion, peeled and quartered
1 jalapeño, stemmed, seeded, and halved
1 poblano pepper, stemmed, seeded, and quartered
2 medium or 3 small tomatillos, husked, cored, and quartered
1 clove garlic, peeled
Canola or olive oil, for drizzling
1 avocado, peeled
¼ cup chopped cilantro
Juice of 1 lime
¼ cup Greek yogurt (or half and half)
4 boneless, skinless chicken breasts
Salt and freshly ground black pepper
Place the onion, jalapeño, poblano, tomatillos and garlic on a rimmed baking sheet. Drizzle with a bit of canola oil and place under a broiler for a few minutes, until the tomatillos are starting to soften and everything is beginning to char. Alternately, char these on the grill. Set aside to cool.
Meanwhile, cook the chicken. Season each breast with salt and pepper. Heat grill to high, wiping down grates with a canola oil coated paper towel. Grill chicken over direct heat until well browned and cooked through, about 5 minutes per side. Alternately, cook the chicken in a grill pan until cooked through.
Transfer the cooled peppers, tomatillos and garlic to the bowl of a food processor. Add the avocado and cilantro and pulse until well combined. Add the yogurt and continue pulsing until the sauce reaches your desired smoothness. Season with salt and pepper to taste.
Spoon sauce over the grilled chicken and serve immediately. The sauce can be stored in an airtight container in the refrigerator for up to 2 days.