Spread one tablespoon pesto on each piece of bread. Place mozzarella cheese on each side of the bread. Then layer a few spinach leaves, a few sun-dried tomatoes, and chicken. Put another bread slice on top.
Butter both sides of sandwich (or brush with olive oil). Then place each sandwich on a panini press, skillet, or griddle. Press sandwiches (or grill on both sides) until warmed through, cheese has melted, and the bread is crisp.
Serve sandwiches with Vegetable Bean Soup.