Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or non-stick spray.
Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package instructions until al dente, about 7-8 minutes. Drain and set aside.
In a large saucepan, melt 3 tablespoons of butter over medium heat. Whisk in the flour and cook for 1-2 minutes until the mixture is lightly golden and bubbling.
Gradually whisk in the warmed milk, ensuring no lumps form. Continue to whisk and cook until the sauce thickens and coats the back of a spoon, about 5-7 minutes.
Remove the sauce from heat and stir in the shredded cheddar cheese and mozzarella cheese until melted and smooth.
Season the cheese sauce with salt, black pepper, garlic powder, and onion powder. Stir well to combine.
In a large mixing bowl, combine the cooked macaroni, cheese sauce, flaked tuna, thawed peas, chopped onion, and chopped celery. Mix gently until evenly combined.
Transfer the mixture to the prepared baking dish and spread evenly.
In a small bowl, mix the breadcrumbs with the melted butter until the crumbs are evenly coated.
Sprinkle the breadcrumb mixture evenly over the top of the casserole.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is bubbly around the edges.
Remove from the oven and let cool for 5 minutes before serving.