Add the boneless skinless chicken breasts, all-purpose flour, salt, and black pepper to a large zip-top bag. Seal the bag and shake to fully coat the chicken with the flour and seasonings. Remove the coated chicken breast from the bag, being sure to shake off any excess flour, and set the chicken on a plate. Discard the bag and excess flour.

In a large 12-inch skillet, on medium-high heat, add the olive oil. Once the oil and skillet are hot, add the chicken breasts and cook for 5-7 minutes on each side or until golden brown. You may need to cook your chicken in two batches to avoid overcrowding your skillet. Once all the chicken has been browned, remove the chicken from the skillet and set aside on a clean plate.

Wipe out any remaining oil and overly browned flour from the skillet using a dry paper towel. This is especially important if you had to cook your chicken in 2 batches.

Turn the heat to medium and add the butter, chopped sweet yellow onion, diced sun-dried tomatoes, minced garlic, fresh thyme leaves, and red pepper flakes. Stir to combine and cook for 1-2 minutes or until the onions are tender.

To the skillet, stir in the chicken stock and heavy cream. Allow the liquids to come to a low boil, then reduce the heat to medium-low and add the grated parmesan cheese and chopped basil. Stir to combine.

Nestle the chicken into the sauce rather than fully submerging it so that the top stays lightly crisp. Allow the chicken to gently finish cooking in the simmering sauce, uncovered for 15-20 minutes. The internal temperature of the chicken, at its thickest part, should reach 165°F when done.

Transfer the chicken to a plate while the sauce reduces.

Increase the heat to medium-high, reduce the sauce for 2-4 minutes, while stirring often to prevent burning, until it coats the back of a spoon. Turn off the heat to the skillet.

Return the chicken to the skillet, spoon some of the sauce over the chicken and garnish with additional chopped sun-dried tomatoes and fresh basil if desired before serving.