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 sauté until translucent, about 5 minutes.
Add the minced garlic to the skillet and cook for 1 minute until fragrant.
Sprinkle the flour over the onions and garlic and stir constantly for 2 minutes to form a roux.
Gradually whisk in the milk, ensuring there are no lumps. Continue to cook and stir until the sauce thickens, about 5 minutes.
Season the sauce with salt, black pepper, dried thyme, and dried basil.
Remove the sauce from heat and stir in the shredded cheddar and mozzarella cheeses until melted and smooth.
In a large mixing bowl, combine the cooked ziti pasta, cheese sauce, flaked tuna, and thawed peas. Mix gently to combine.
Transfer the mixture to a greased 9×13-inch baking dish and spread evenly.
In a small bowl, mix the breadcrumbs with the grated Parmesan cheese. Sprinkle this mixture evenly over the casserole.
Bake in the preheated oven for 25 minutes, or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let it rest for 5 minutes before serving.