To a large Dutch oven or large skillet, add in 2 tablespoons of the sun-dried tomato oil and heat over medium-high heat. Sauté the chicken breasts that have been seasoned with salt and pepper. Cook on each side for 5-6 minutes or until the internal temperature reached 165 degrees. Transfer to a cutting board and cut into bite sized pieces. Transfer to a bowl and cover to keep warm.
To the same pan, and the remaining oil and heat over medium-high heat. Add in the onion and sauté for 5-6 minutes or until onion becomes translucent. Add in the garlic and sauté for 1 more minute. Transfer mixture to same bowl with chicken. Cover to keep warm.
To the same pan, melt the butter. Add in the flour and whisk together. Mixture will bubble and brown slightly. Cook for about 2 minutes. Scrape up any brown bits from the chicken and onions. Pour in the stock, milk, Italian herbs, garlic salt, pepper and orzo pasta. Stir together. Heat over medium-high heat and bring to a boil. Reduce heat to medium-low and simmer for 6-8 minutes. Stir often. Mixture will begin to thicken and orzo will begin to plump up.
Once mixture begins to thicken, add in the cooked chicken and onion mixture, chopped spinach, sun-dried tomatoes, Parmesan cheese, lemon juice and mustard. Stir well. Continue to cook for another 2 minutes.
Serve right away with additional Parmesan cheese for topping.