Grate garlic and ginger on a microplane into a large bowl. Combine with yogurt, olive oil, garam masala, and salt.
Add chicken pieces to the bowl, toss to coat with marinade, and cover and refrigerate for 2 to 24 hours.
When ready to cook, pull out chicken from fridge an hour before putting in the oven and let come to room temperature. Preheat oven to 400 degrees F.
Meanwhile, slice onions into 1/4" slices and arrange on a rimmed baking sheet. Place the marinated chicken on top in a single layer, and discard remaining marinade.
Bake until the internal temperature on the chicken reaches 165 degrees F. (Use an instant read thermometer). Serve chicken with onion slices and use a spoon to drizzle pan juices around. Alternatively, you may pour onion and juices from the pan into a blender and run on high for about one minute until a smooth sauce is formed. Serve chicken drizzled with sauce. Roughly chop cilantro leaves and sprinkle over top for garnish.