Preheat your oven to 375°F and lightly grease an 8×11 casserole dish.
Bring a large pot of salted water to a boil. Cook the pasta until al dente according to package instructions, then drain.
While the pasta is cooking, combine cream cheese, mayonnaise, ¼ cup shredded cheese, garlic powder, salt, and pepper in a large mixing bowl.
Add the hot, drained pasta to the mixing bowl and stir until everything is well-coated.
Mix in the drained tuna and fresh spinach until evenly combined. The warm pasta will wilt the spinach slightly.
Transfer everything to your prepared casserole dish. Sprinkle the remaining ½ cup shredded cheese evenly on top.
Bake for 15 minutes. Switch to the broiler for 2-3 minutes to achieve that gorgeous crispy topping.
Serve warm straight from the oven, and enjoy!
