Preheat the oven to 375°F (190°C).
Cook the ziti pasta in a large pot of boiling salted water according to package instructions until al dente, about 8-10 minutes. Drain and set aside.
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk continuously for 2 minutes to form a roux.
Gradually whisk in the milk, continuing to whisk until the sauce thickens and is smooth, about 5-7 minutes.
Remove the sauce from heat and stir in salt, black pepper, oregano, and basil.
In a large skillet, heat olive oil over medium heat. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for 1 minute more.
In a large mixing bowl, combine the cooked pasta, sautéed onion and garlic, flaked tuna, thawed peas, half of the mozzarella cheese, half of the Parmesan cheese, and the prepared white sauce. Stir gently to combine.
Transfer the mixture to a greased 9×13 inch baking dish. Sprinkle the remaining mozzarella and Parmesan cheese evenly on top.
Bake uncovered in the preheated oven for 20-25 minutes, or until the casserole is bubbly and the cheese on top is golden brown.
Remove from the oven and let it cool for 5 minutes before serving.