In a large non-reactive skillet over medium heat and melt in 1 Tbsp butter and 1 Tbsp olive oil. Season chicken all over with about 1/2 salt, 1/4 tsp black pepper and 1/2 tsp garlic powder. Add chicken to the hot skillet and sauté 3-5 minutes per side, or until cooked through to a safe internal temperature of 165 ̊F on an instant-read thermometer.
Remove chicken from the pan and transfer to a cutting board to cool then slice chicken if desired.
In the same pan over medium heat, add 1 Tbsp oil and sliced mushrooms, and sauté 5-7 minutes, stirring occasionally until lightly golden and excess liquid has evaporated. Add 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.
Stir in cream and sprinkle 1/2 cup parmesan cheese over the top. Bring to a light boil, stirring frequently then reduce heat and simmer (2 minutes) until sauce is slightly thickened. Season sauce to taste with salt and pepper.
Add spinach and stir just until wilted. Add chicken back to the pan, and spoon warm sauce over it then remove from heat. Garnish with chives if desired.