Chicken Tacos with Cilantro Lime Ranch Dressing

boneless, skinless chicken breasts, about 8 ounces each

medium white onion, chopped

head iceberg lettuce, shredded

taco sized tortillas

tomatillos, husks removed

large jalapeno

garlic

zest and juice of 1 lime

packed cup cilantro, leaves and stems OK

packet Hidden Valley Ranch Dressing Mix, or 2 tablespoons homemade ranch dressing mix

non fat plain greek yogurt

milk

salt and pepper