Preheat the outdoor grill to medium heat; for charcoal, arrange an even layer of hot coals for consistent cooking.
Place the frozen turkey burgers on the grill, cover, and cook for about 6 minutes.
Flip the burgers, cover, and cook for another 4 minutes.
Remove the cover, place a slice of cheddar on each burger, and cook uncovered for about 2 more minutes until the cheese melts and the burgers reach a safe internal temperature (follow package guidance or use a meat thermometer).
If desired, place the hamburger buns cut-side down on the grill for a minute or two until lightly toasted.
To assemble, spread 1 to 1 1/2 tablespoons (or more to taste) of ranch dressing on each bun, top with a turkey burger, then add lettuce, tomato, onion, pickles, and any other desired condiments such as ketchup, mustard, relish, or BBQ sauce.
Serve the assembled burgers immediately.
