Grill the chicken for about 6-7 minutes on each side or until fully cooked. Let it rest for a few minutes before slicing it into thin strips.
Drizzle one side of each slice of sourdough bread with olive oil.
Spread 1 tablespoon of pesto sauce on the un-oiled side of each slice of bread.
Layer the grilled chicken strips, mozzarella cheese slices, and fresh spinach leaves on one slice of bread.
Top with the other slice of bread, with the pesto side facing down towards the filling.
Heat a skillet over medium heat and grill the sandwich on both sides until the bread is golden brown and the cheese is melted, about 3-4 minutes per side.
Let the sandwich rest for a minute before slicing it in half. Enjoy!