In a large cast-iron skillet, heat the butter and olive oil over medium-high heat until shimmering and fragrant.

Generously season the chicken tenderloins with salt and pepper.

Add the chicken to the hot skillet and cook for about 5-6 minutes until golden brown on both sides. It doesn't need to be fully cooked at this stage. Remove from the skillet and set aside.

Stir in the diced onion and sauté for about 3 minutes until softened. Add the minced garlic and continue to cook for another 2 minutes until fragrant.

Pour in the chicken broth and allow it to reduce by half, scraping any tasty bits from the bottom. Stir in the heavy cream and Italian seasoning, bringing it to a gentle simmer.

Place the chicken back in the skillet and simmer for a few minutes until the sauce thickens slightly and the chicken is cooked through.

Reduce the heat to medium-low and mix in the Parmesan cheese, roasted red peppers, pepperoncini, and crushed red pepper flakes. Stir until the cheese melts and everything is beautifully combined.

Check for seasoning and add salt and pepper as needed.