Strip the kale leaves from the ribs and chop the leaves into bite sized pieces. Add the freshly chopped kale and walnuts to a food process and pulse for about 20 seconds. Add in the parmesan and garlic and pulse again. It should be a rough mixture. With the food processor running, begin to drizzle the olive oil in slowly. The mixture will start to smooth out. To finish the pesto, stir in the lemon juice, salt, and pepper. 

While waiting for your panini press to heat up (can alternatively use a medium sized pan on the stovetop), slice the tomatoes into thin sandwich slices. Cut the loaf of bread into 4 equal pieces before slicing each in half to create the sandwich bread. Add a layer of pesto to both sandwich halves of each sandwich. Layer in the mozzarella, tomato slices, and some spinach leaves.

Take a little bit of olive oil and brush it on the outside of both sides of the sandwich before adding it to the panini press or pan. If using a panini press, each sandwich should take about 3-5 minutes. If using the stovetop, each sandwich should be flipped halfway and take about 5 minutes on low-medium heat. The sandwich will be done when golden and crispy on the outside and the mozzarella is melted. Serve while hot and enjoy!