Heat the oven to 400 degrees F. Make a slit two-thirds of the way through the center of each salmon fillet making sure not to cut all the way through. Season each fillet with the salt and pepper. In a bowl, combine the spinach, pesto, tomatoes, and pine nuts. Spoon 1/3 cup of the mixture into each slit.
Arrange the fillets on a broiler pan coated with cooking spray. Roast for 8 to 10 minutes or until the spinach mixture is heated through.