Pat the chicken dry with paper towel and season the chicken on both sides with salt. Set aside.
Place the olive oil and butter in a large cast iron pan and place over medium-high heat. When hot, add the chicken (skin side down). Let cook for about 5 minutes or until the chicken has browned nicely. Using tongs, carefully turn the chicken and cook on the other side for an additional 5 minutes. Remove the chicken from the pan and place on a plate near your work station.
Place the pan back over the heat and add the orzo, using a spatula, mix to coat and cook until the pasta begins to brown, about 2 minutes. Reduce the heat to medium, add the chopped onions and cook for an additional 2-3 minutes until softened. Add the garlic and cook 30 seconds longer.
Place the seared chicken in the pan with the orzo. Add the chicken stock, tomatoes, salt, and pepper. Increase the heat back to medium-high and bring to a boil. Once boiling, lower the heat and bring to a simmer. Cover and cook until the orzo has almost absorbed all the liquid and the chicken is cooked through about 15-20 minutes.
Remove the pan from the heat and swirl in the pesto. Cover again and allow to sit for about 5 minutes. Garnish with parsley (if using) and serve directly from the pan.