Preheat the oven to 350°F (175°C).
Bring a large pot of salted water to a boil. Add the ziti pasta and cook according to package instructions until al dente, about 8-10 minutes. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic to the skillet and cook for 1 minute until fragrant.
In a large mixing bowl, combine the cooked pasta, sautéed onion and garlic, flaked tuna, cream of mushroom soup, sour cream, thawed peas, salt, and black pepper. Stir until well combined.
Add half of the shredded mozzarella cheese and half of the grated Parmesan cheese to the mixture and stir to combine.
Transfer the mixture to a greased 9×13-inch casserole dish and spread evenly.
Sprinkle the remaining mozzarella and Parmesan cheeses evenly over the top.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the cheese is melted and golden brown.
Remove from the oven and let cool for 5 minutes.
Garnish with chopped fresh parsley before serving.