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 skillet, melt the butter over medium heat. Add the chopped onion and minced garlic and sauté until the onion is translucent, about 3-4 minutes.
In a medium saucepan, combine the condensed cream of mushroom soup, Velveeta cheese cubes, and milk. Heat over medium-low heat, stirring frequently, until the cheese is completely melted and the mixture is smooth.
In a large mixing bowl, combine the cooked macaroni, sautéed onion and garlic, melted cheese sauce, flaked tuna, thawed peas, salt, and black pepper. Stir gently until all ingredients are well incorporated.
Transfer the mixture to a 9×13-inch baking dish. Sprinkle the shredded cheddar cheese evenly over the top, then sprinkle the crushed potato chips on top of the cheese.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it cool for 5 minutes before serving.