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 3 tablespoons of unsalted butter over medium heat.
Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly to form a roux.
Gradually whisk in the whole milk, continuing to whisk until the mixture thickens and becomes smooth, about 5 minutes.
Remove the saucepan from heat and stir in the shredded sharp cheddar cheese until melted and smooth.
Add salt, black pepper, garlic powder, and paprika to the cheese sauce and stir to combine.
In a large mixing bowl, combine the cooked macaroni, cheese sauce, flaked tuna, thawed peas, and chopped onion. Mix gently until evenly combined.
Transfer the mixture into a 9×13-inch baking dish and spread it out evenly.
In a small bowl, mix the crushed buttery crackers with 2 tablespoons of melted butter.
Sprinkle the buttery cracker topping evenly over the casserole.
Bake in the preheated oven for 20-25 minutes, or until the topping is golden brown and the casserole is bubbly.
Remove from the oven and let cool for 5 minutes before serving.