Preheat the smoker to 250°F (121°C)
In a large bowl, add ground turkey, garlic, red onion, salt and black pepper. Use your hands to mix it all together. Be careful not to overwork.
Divide the mixture into 4 equal balls. Use your hands to press into a hockey puck/patty shape.
Place the burgers on the smoker grates and cook until the internal temperature hits 160°F (71°C), about 1 hour
Once cooked, remove the patties from the smoker. Place a slice of mozzarella cheese on top and set to one side to rest.
Whilst the burgers are resting, prepare the buns. Spread ½ tbsp mayonnaise and ¼ tsp Dijon mustard on both halves of each bun.
Slide the cheese-topped burgers onto each bottom bun. Top with a slice of tomato and a lettuce leaf, before crowning with the top bun to serve.