First, prepare the pesto chicken. In a small bowl, toss thinly sliced chicken with extra virgin olive oil and 1 tbsp of the pesto until well-coated. Sauté in a skillet over medium-high heat for about 6 to 8 minutes, or until cooked through.
Meanwhile, preheat a cast iron griddle over medium heat.
Next, assemble the panini. Spread about 1 tsp butter on the outside of each slice of sourdough bread, then carefully place one slice of bread butter side down on a plate. Spread the remaining pesto onto the bread, then top with mozzarella cheese slices. Layer with spinach leaves, then the cooked pesto chicken, then top with shredded mozzarella cheese. Close the sandwich with the remaining slice of bread (butter side up).
Carefully transfer the panini to the hot cast iron griddle and cook until bottom is crisped and golden-brown, about 3 to 4 minutes. Carefully flip and cook until the bottom is crisped and golden-brown again, another 3 to 4 minutes. (Alternatively, you can use a preheated grill press on top of the sandwich to crisp both sides at once and avoid flipping your panini.)
Transfer panini to a cutting board and slice diagonally in half. Serve immediately.
Notes
Pesto: Use your favorite store-bought pesto or try our nut-free basil pesto recipe here for a delicious homemade alternative.Fresh mozzarella: Thinly sliced fresh mozzarella works best for paninis. The thinner the slice, the quicker it melts - meaning an extra gooey and delicious panini!Cooking methods: If you have a panini press, you can easily toast this sandwich on it in about the same amount of time as it takes on the stove. Alternatively, cook the sandwiches on a cast iron griddle and optionally use a cast iron grill press to toast both sides at once.