Mix Everything Together: In a large mixing bowl, combine the cooked pasta, drained tuna, mayonnaise, diced onion, Old Bay seasoning, salt, and black pepper. Stir well to evenly coat everything.
2 12-ounce cans tuna, drained, 1 ½ cups mayonnaise, 1 medium onion, 1 teaspoon Old Bay seasoning, ½ teaspoon salt, ¼ teaspoon black pepper