Slice each breast in half lengthwise, so you have 4 thinner breasts.
Season each breast on each side with salt and pepper and dredge in flour, shaking off any excess.
Melt 2 tablespoons butter to a skillet over medium heat.
Add coated chicken and sear for about 3 minutes on each side.
Remove chicken from skillet and set aside.
Melt remaining butter and add onion and garlic. Sauté for 5 to 6 minutes or until onions just begin to cook down. Season with salt and pepper.
Add sun-dried tomatoes and continue to sauté for an additional 3 to 4 minutes.
Stir cream into skillet and simmer until cream reduces slightly, about 4 minutes.
Stir Parmesan into cream mixture until smooth creamy.
Add spinach to skillet and stir until spinach leaves have just wilted. Season with salt and pepper.
Add chicken back into skillet, spoon some sauce over each pieces and simmer for 3 to 4 minutes. Serve.
