Preheat oven to 400* F.
In a small bowl, combine the finely chopped thyme and rosemary, lemon zest, 2 tablespoons of the olive oil, and a generous pinch of salt and several grinds of freshly cracked black pepper. Mix together to incorporate all of the ingredients. Use at least half of the mixture to stuff under the chicken thigh skin, and rub the rest all over the tops of the chicken skin. Season with a few more generous pinches of salt and pepper.
In a 10" or 12" cast iron skillet, warm the remaining tablespoon of olive oil over medium heat. Once the pan is really hot, place the chicken, skin side down in the skillet and sear until the skin is golden brown and crispy, about 5 to 8 minutes depending on the size of the chicken pieces.
Once the skin-side of the thighs are seared, remove the chicken from the pan, and drain some of the fat out. Scatter the sliced onions, lemon slices, and garlic cloves in the pan. Place the chicken skin-side UP on top of the onions/lemons/garlic.
Gradually pour chicken broth in the skillet so it just submerges the bottom of the chicken thighs, you might not need all 3/4 cups of the broth depending on the size of your pan, so pour slowly, making sure you keep the crispy skin above the liquid.
Finish cooking the chicken in your preheated oven, about 20-25 minutes, or until a meat thermometer registers at 165* F. Remove from oven, and let the chicken rest in the skillet for 5 minutes before serving. You could toss some greens with a simple vinaigrette and toast up some baguette slices while the chicken is roasting, or simply on it's own. Enjoy!