Start by placing your chicken breasts on a cutting board and carefully slicing them horizontally through the center. Stop just before you cut all the way through so you can open the meat like a book. This creates the pocket necessary for holding all that delicious stuffing.
Place your butterflied chicken breasts into a bowl or ziplock bag and pour the marinade over them, ensuring they are well coated. Let them sit for 30 minutes to allow the flavors to penetrate the meat deeply. This step also helps tenderize the chicken for a better texture.
While the chicken is marinating, take a medium mixing bowl and combine the softened cream cheese, chopped spinach, sun-dried tomatoes, shredded cheese, and seasoning. Stir everything together until you have a uniform, thick paste where the ingredients are evenly distributed.
Open up each marinated chicken breast and divide the cheese and spinach mixture evenly among them, placing it in the center. Fold the top flap of the chicken back over the filling and secure the open edges with toothpicks to keep everything inside.
Preheat your grill to medium-high heat and place the chicken breasts directly onto the grates. Cook them for about 10 minutes total, flipping them carefully halfway through the cooking time. The chicken is ready when the internal temperature reaches 165°F.
While the chicken finishes grilling, melt the butter in a saucepan over medium-high heat and whisk in the heavy cream, sun-dried tomato oil, parmesan, and seasoning. Bring the mixture to a boil, then reduce the heat and let it simmer gently for 5 minutes until thickened.
Remove the toothpicks from the grilled chicken breasts and arrange them on a serving platter. Pour the hot, savory cream sauce generously over the chicken and serve immediately while warm.