Slice a small incision at the thickest part of each chicken thigh.
In a small bowl, whisk together the yogurt, all seasonings, grated ginger, and grated garlic. Massage the mixture into each chicken thigh. Place the chicken in a covered bowl or plastic bag, and marinate in the refrigerator for 30 minutes.
Preheat the oven to 400 F. Arrange one rack in the middle of the oven, and one on the top rack under the broiler.
Remove the chicken from the bag or bowl, and thread them onto skewers. Place the skewered chicken on a baking sheet or casserole dish, and place on the middle rack of the oven. Roast for 15 minutes, rotating halfway through, until the chicken is no longer pink.
Once the chicken is opaque white, heat the broiler to high, or 525 F. Brush the chicken with melted butter. Move the chicken to the top rack under the broiler, and roast until crispy and beginning to blacken on the edges, about 10 minutes.
Remove the chicken from the broiler, and remove from skewers to serve. Serve with onions and lemon.