Preheat the oven to 350°F (175°C). Grease a 9×13 inch baking dish.
Cook the elbow macaroni according to package instructions until al dente. 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 it forms a smooth paste and is lightly golden.
Gradually whisk in the milk, ensuring no lumps form. Continue to cook, stirring constantly, until the sauce thickens, about 5-7 minutes.
Remove the sauce from heat and stir in the shredded cheddar and mozzarella cheeses until melted and smooth.
Season the cheese sauce with salt, black pepper, garlic powder, and onion powder.
In a large mixing bowl, combine the cooked macaroni, cheese sauce, flaked tuna, thawed peas, chopped onion, and celery. Mix gently to combine.
Pour the mixture into the prepared baking dish and spread evenly.
In a small bowl, mix the breadcrumbs with the melted butter until evenly coated.
Sprinkle the buttered breadcrumbs evenly over the casserole.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let cool for 5 minutes before serving.