Preheat a large, deep skillet or shallow pot over medium heat.
Season both sides of the chicken with salt and pepper.
Add the oil. When the oil is hot, add the chicken. Cook until golden brown, approximately 5 minutes. If too hot, reduce the heat.
Flip the chicken over and cook an additional 5 minutes until golden brown and also cooked all the way through. If your chicken is browning too fast, reduce the heat to slow browning but continue cooking the chicken. The chicken is done when it is 165F in the thickest part.
Remove the chicken from the pan and set aside. Leave all of the oil and drippings in the pan. Add the butter to the same pan you cooked the chicken in over medium heat and let the butter melt.
Add the onion and cook until translucent, approximately 3 minutes. Add the garlic, stir, and cook for 30 seconds until fragrant.
Add the white wine to the onions. Cook until reduced by half, approximately 2 minutes.
Mix the cornstarch with 1 Tablespoon of water and stir to combine. Add the cornstarch to the onion mixture. Also add the milk, sundried tomatoes, spinach, 1/4 teaspoon salt, and 1/4 teaspoon black pepper. Stir and cook until the sauce thickens, approximately 5 minutes.
Remove the sauce from the heat. Add the Parmesan cheese and stir until melted. At this point, season the sauce to taste with salt and black pepper.
Put the chicken back in the sauce and coat, if desired. Serve over pasta or with vegetables.