Preheat oven to 400F.
Place the chicken breasts in a large ziploc bag. Add in the dijon mustard, 1 tsp. cumin, 1 tablespoon olive oil, 1/2 tsp. salt, 1/2 tsp. black pepper, and zest & juice of 1 lemon. Use your fingers to rub the seasonings into the chicken.
In a cast-iron skillet (or oven-safe stovetop pan) over medium high heat, add 1 tablespoon olive oil. Add the sliced red onion and cook for 1 minute. Push the onions to the edges of the skillet.
Add the chicken breasts into the pan to sear each side for about 3-4 minutes.
Turn off the stovetop or remove from heat. Add in the chicken broth and juice of the remaining 3 lemons. Stir in the chopped parsley. Evenly add in the orzo, then stir or press down so the chicken broth covers it. Sprinkle the remaining 1/2 tsp. salt and 1/2 tsp. black pepper on top.
Tightly cover with aluminum foil. Bake for 20-25 minutes, until the orzo is tender.