Cut each chicken breast in half, horizontally; so you end up with 4 cutlets. Season both sides of the chicken with about 1/2 tsp salt, 1/4 tsp black pepper and 1/2 tsp garlic powder.
In a large skillet, heat 1 Tbsp butter and 1 Tbsp olive oil over medium heat. Add the seasoned chicken to the hot skillet and sauté until cooked through, about 3-5 minutes per side. Chicken should be cooked until it reaches an internal temperature of 165° F.
Remove chicken from skillet; cover to keep warm. If desired, slice each breast.
Prepare the Tuscan Sauce
In the same skillet the chicken was cooked in, add the remaining 1 Tbsp. olive oil and the sliced mushrooms; sauté 5-7 minutes, until golden and excess liquid has evaporated; stirring occasionally and scraping up any browned bits. Add the chopped sun-dried tomatoes, green onion, minced garlic 1/2 tsp salt and 1/4 tsp black pepper. Sauté another 1 minute or until garlic is fragrant, be careful not to brown the garlic.
Stir in the half-and-half and chicken broth; sprinkle in the parmesan cheese. Heat just to a light boil, stirring frequently; reduce heat and simmer about 2 minutes or until sauce thickens slightly. Season with salt and pepper to taste.
Add the baby spinach into sauce and stir just until it's wilted. Add the cooked chicken back to the pan, along with any juices. Gently spoon some of the sauce over the chicken, then remove from heat. Garnish with chives or sliced green onion, if desired.