Preheat the oven to 350°F (175°C).
Cook the elbow macaroni according to package instructions until al dente. Drain and set aside.
In a large saucepan, melt the butter over medium heat.
Whisk in the flour and cook for 1-2 minutes until lightly golden, stirring constantly.
Gradually whisk in the milk, continuing to whisk until the sauce thickens and bubbles, about 5 minutes.
Remove the saucepan 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.
Add the drained tuna, thawed peas, and chopped onion to the cheese sauce. Stir gently to combine.
Fold the cooked macaroni into the sauce mixture until evenly coated.
Transfer the mixture to a greased 9×13-inch baking dish and spread evenly.
In a small bowl, toss the breadcrumbs with olive oil until evenly coated.
Sprinkle the breadcrumb mixture 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.