Preheat oven to 350F. Heat a cast iron (or other oven-safe) skillet over medium-high heat. Add the butter and let it melt.

Season the chicken thighs with salt and pepper, then lay them in the skillet.

Let the chicken thighs brown for several minutes, until they are golden, before flipping over and cooking the other side.

In a food processor blend the basil leaves, olive oil, and pine nuts. Season with additional salt.

Turn off the heat under the skillet. Spoon the pesto sauce over the chicken thighs.

Layer with the tomato and mozzarella.

Bake for about 30 minutes, or until the chicken is cooked all the way through and the cheese is melted and browned.