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 1 tablespoon of 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 mixture thickens and comes to a simmer, about 5-7 minutes.
Remove the sauce from heat and stir in the shredded cheddar cheese 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, tuna, peas, celery, and onion.
Pour the cheese sauce over the macaroni mixture and stir gently to combine.
Transfer the mixture into a greased 9×13 inch baking dish.
In a small bowl, mix the crushed potato chips with the melted butter.
Sprinkle the potato chip topping evenly over the casserole.
Bake in the preheated oven for 25 minutes, or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it rest for 5 minutes before serving.