Line a small baking sheet with parchment paper or foil & set aside. Gather & prep all ingredients. To grate the onion, arrange a few paper towels on a cutting board, set a box mixer on top, & run the onion along the sides of the box mixer to grate it into fine, shredded pieces. Once grated, use the paper towel to soak up excess moisture from the onion. Set aside.
Add the ground turkey, grated onion, garlic, mayonnaise, & Worcestershire sauce to a large bowl. Season with 1 heaping teaspoon kosher salt & ground black pepper as desired. Use your hands to mix until just combined – overworking the turkey burger mixture leads to tough burgers; take care not to overmix.
Divide the mixture into 4 equal portions, forming each into a 1-inch thick burger patty. If needed or desired, spritz a little bit of olive oil on your hands to make handling the burger patties easier. The mixture will be very wet – this is expected & intended! There’s no need to fuss over making them look perfect at this point – just aim for forming them into the general shape & size you’re after. Set them on the prepared baking sheet. Transfer the baking sheet with the turkey burger patties to the freezer. Chill for at least 20 minutes.
Meanwhile, as the turkey burger patties chill, prep the grill. Prepare the grill for medium-high direct heat grilling, about 450-500 degrees F. Once preheated, clean the cooking surface by firmly brushing a wire brush over the grates. Just before placing the burgers on the grill, generously spritz a wad of paper towel with nonstick cooking spray. Use grill tongs to run the oiled paper towel over the grill grates, generously oiling the grates to prevent sticking.
Remove the turkey burger patties from the freezer. Finish forming them into 1-inch thick patties, using your thumb to create an indent in the center of each patty. Press each turkey burger patty down onto the prepared grill grates. Grill the turkey burgers for 6-7 minutes per side, or until a thermometer inserted in the center of the turkey burgers registers 165 degrees F. If you’d like to add cheese to your turkey burgers, do so when there are 2 minutes left of grilling time.