Make the Frisco Sauce: In a small bowl, mix Thousand Island dressing and ketchup until smooth. Set aside.

Cook the Patties: Form ground beef into thin patties. Season with salt and pepper. Cook in a hot skillet over medium-high heat for 2–3 minutes per side until browned.

Assemble & Toast: Butter one side of each bread slice. On the unbuttered side, layer American cheese, beef patty, Swiss cheese, and Frisco sauce. Top with another slice of bread, buttered side out. Grill on medium heat until golden and cheese is melted, flipping once.