Begin by boiling water in a large pot over high heat. Add the rotini pasta and cook according to the package instructions until it's al dente, usually takes about 8-10 minutes. Drain and rinse under cold water.

In a large skillet, heat a splash of oil over medium-high heat. Add the ground beef and cook until browned, about 7-10 minutes. Season with salt and pepper, drain excess fat, and let cool.

Wash and dice the tomatoes, and finely chop the red onion. Rinse and chop the lettuce into bite-sized pieces and store in the fridge until serving.

In a large mixing bowl, combine the cooled rotini pasta, browned beef, shredded cheddar cheese, diced tomatoes, and red onion. Gently fold together. Pour in the Thousand Island dressing and mix until coated.

Just before serving, fold in the chopped lettuce. Cover and refrigerate for at least 30 minutes to meld flavors.