A No. 10 or 11 skillet will accommodate all of the chicken. Or use two small skillets.
Preheat oven to 400 degrees. In a small bowl, combine the chili powder, cumin, and paprika, plus salt and pepper to taste. Rub chicken all over with the chili mix.
Add olive oil to a large cast-iron or other ovenproof skillet, and swirl to cover bottom. Add onion rounds in a single layer, then thyme. Add chicken, skin side up. Transfer to oven, and bake until the chicken has cooked through, 30 to 40 minutes.
Let chicken cool slightly. Using your fingers, remove the bones. Slice each breast, crosswise, into four or five pieces. Serve with the roasted onion rounds and thyme.