In a mixing bowl, combine buttermilk, mayonnaise, and sour cream. Stir in ranch seasoning, Dijon mustard, and lemon juice. Add minced garlic, chopped onion, and sun-dried tomatoes. Sprinkle in crushed red pepper flakes and mix well. Chill in the fridge for at least 15 minutes.

Thinly slice the chicken thighs and season with kosher salt, black pepper, garlic powder, smoked paprika, and chili powder. Heat olive oil in a skillet over medium-high heat. Add seasoned chicken and cook for 6–8 minutes until browned and cooked through.

Butter the cut sides of the hoagie rolls. After the chicken is cooked, lower skillet heat to medium and toast rolls face-down for about 1–2 minutes until golden brown.

Lay the cooked chicken inside each toasted hoagie roll. Drizzle chilled ranch sauce over the chicken and top with provolone cheese. Melt cheese under a broiler for 1-2 minutes or cover the pan until melted.

Serve the cheesesteaks hot with extra ranch sauce on the side for dipping.