Slice the chicken into thin strips and generously season them with sea salt and ground pepper on every side.
Heat the butter in a large heavy skillet over medium-high heat, then add the chicken in a single layer and cook until golden brown on all sides, about 8 minutes. Cook in batches if necessary. The chicken does not need to be fully cooked yet. Remove from the pan and set aside.
In the same skillet, add the diced onion and garlic and sauté over medium-high heat until they become fragrant and lightly browned, approximately 5 minutes.
Pour in the chicken stock and add the sliced roasted red peppers. Allow the liquid to simmer and reduce by half, which should take around 5 minutes, intensifying the flavors.
Turn off the heat. Gradually whisk in the heavy cream and shredded Parmesan cheese until the sauce turns smooth and creamy with no lumps.
Turn the heat back to medium-high and stir in the Italian seasoning along with the drained pepperoncini peppers. Taste and add more salt if needed.
Return the browned chicken strips to the skillet and continue to cook until the chicken is fully cooked through and the sauce thickens to a creamy consistency that coats the chicken well. If the sauce thickens too much, thin it out with additional chicken stock, adding one tablespoon at a time.
Serve the creamy pepperoncini chicken hot over your favorite rice, pasta, or roasted veggies. Optionally, garnish with extra Parmesan and fresh parsley for added flavor and color.
