1. Preheat oven to 350ºF. Defrost and heat spinach in small amount of water in saucepan; drain thoroughly. While spinach is warming, combine ricotta, sour cream, flour, 1 cup Parmesan or Asiago cheese, and tuna in large bowl.
2. Add eggs, drained spinach, salt, and pepper, and mix thoroughly. Coat 9 1/2" by 5 1/2" by 3" loaf pan with nonstick cooking spray. Pour mixture into pan.
3. Sprinkle remaining 1/4 cup grated cheese evenly over top. Bake 55 minutes to an hour, until top is golden. Let cool slightly, slice and serve.