Begin by bringing a large pot of salted water to a boil. Add the rotini pasta and cook until al dente, about 8–10 minutes. Drain and rinse under cold water. Set aside to dry.

In a large skillet, heat over medium-high. Add ground beef, cook for 6–8 minutes, breaking apart until browned. Season, drain excess fat, then let cool.

Wash and finely dice the tomatoes and red onion. Chop the lettuce into bite-sized pieces and keep chilled until ready to assemble.

In a large mixing bowl, combine cooled rotini, browned beef, cheddar cheese, diced tomatoes, and red onion. Toss gently to combine.

Pour in the thousand island dressing and stir until ingredients are well-coated. Adjust seasoning to taste.

Just before serving, fold in the chopped lettuce. Chill in the refrigerator for at least 30 minutes for best flavor.