Heat a cast-iron grill pan over medium-high heat. Coat the chicken breast with olive oil, salt, and pepper. Cook for 4-5 minutes per side until dark grill marks form and the internal temperature reaches 165°F (74°C). Let rest, then slice into thick strips.

Spread butter evenly on one side of each thick slice of artisan sourdough. Toast butter-side down in a skillet over medium heat for 2-3 minutes until a deep golden-brown, crispy crust forms.

Place the toasted bread on a baking sheet, toasted side down. Arrange the chicken strips on the bottom slice, then fan out the avocado slices in overlapping layers over the warm poultry.

Drape the provolone and sprinkle the mozzarella over the avocado. Place under the broiler on high for 1-2 minutes until the cheese is gooey with slight blistering and golden-brown spots.

Remove from the oven and immediately garnish the melted cheese with red pepper flakes and chopped cilantro. Top with the remaining sourdough slice slightly askew to reveal the cheese pull.