Prepare a medium-hot fire in a charcoal grill with a cover, or preheat a gas grill to medium-high.
To make the mayonnaise, combine all the ingredients in a bowl and whisk to blend well. Cover and refrigerate until assembling the burgers.
To make the relish, heat the oil in a fireproof skillet on the grill until hot but not smoking. Add the onion and cook until soft and caramelized. Remove from the grill and allow to cool. Place the bell peppers and corn on the grill and cook until charred on each side. Remove the peppers to a bowl, cover with plastic wrap, and set aside for 10 minutes. Let the corn cool until you can handle it easily. Peel the peppers over a clean bowl, catching any juices. Seed the peppers, cut into a rough 1/4-inch dice, and transfer to the bowl with their juices. Husk and silk the corn, slice the kernels from the cob, and add them to the peppers. Chop the onions into a rough dice, reserving 1/2 cup for the patties, and add the remaining to the pepper and corn mixture. Add the tomatoes, parsley, and salt and toss. Cover and refrigerate until assembling the burgers.
To make the patties, heat the oil in a fireproof skillet on the grill until hot but not smoking. Add the carrots and mushrooms and sauté until soft, 5 to 7 minutes. Remove from the heat and let cool. Place the turkey in a large bowl. Add the onion, egg, bread crumbs, mustard, garlic, salt, pepper, and the sautéed vegetables. Using a fork and taking care not to compact the meat, mix until the ingredients are thoroughly incorporated and evenly distributed throughout. Form into 6 equal patties to fit the buns. Cover and refrigerate until ready to grill.
Brush the grill rack with vegetable oil. Place the patties on the rack, cover, and cook, turning once, until the juices run clear, 5 to 7 minutes on each side. During the last 2 minutes of cooking, place the buns, cut side down, on the outer edge of the grill rack to toast lightly. During the last minute of grilling, top each patty with 1 slice of the cheese.
To assemble the burgers, spread a generous amount of the mayonnaise over the cut sides of the buns. On each bun bottom, place equal portions of the spinach, a patty, and a generous portion of the relish. Add the bun tops and serve.
Makes 6 burgers
Suggested Wine Pairing: Merlot
Contestant Comments: Although there are many steps to this burger, they are all simple and produce a fabulous tasting meal. Our dinner guests are always impressed with the moist and flavorful qualities of this turkey burger. If desired, the relish and mayonnaise can be made the night before.
Amy Green, Carollton, Texas