Add the garlic, onion, turkey, mustard, soy sauce, and salt and pepper, to taste, to a large mixing bowl. Mix well with a wooden spoon. Divide the turkey into 8 equal-sized balls and flatten them into patties. Lay 2 slices of cheese on 4 of the patties and cover each with another patty. Pinch the edges to seal the patties closed, so the cheese will not melt out.
Season the tomatoes with salt and pepper, to taste, and brush with olive oil. Grill for 2 minutes per side. Remove from the grill to a plate. Toast the buns on the grill and set aside.
Grill the burgers until cooked through, about 5 to 6 minutes per side. Transfer the burgers to the buns and top with Spicy Mayo Sauce, lettuce and grilled tomatoes.
Spicy Mayo Sauce:
Combine all the ingredients in a small bowl.
With your finger, press down on the center of the patty. This will keep the juices locked in and prevents the burger from puffing up.