Stir together mayonnaise and ketchup in a small bowl until smooth. Set aside.
Season burger patties with salt and pepper. Heat a large skillet or grill pan over medium-high heat. Add patties in batches and cook to desired doneness, about 3-4 minutes per side.
Spread mayonnaise mixture over each slice of Texas toast. Top half with a burger patty, a slice of cheese, 2 slices of tomato, shredded lettuce and top slice of bread.
