1. In a medium skillet heat the oil over medium heat. Add the onion and sauté until translucent (not brown), about 4 to 5 minutes. Add the garlic and cook until fragrant, about 1 minute (being careful not to burn the garlic). 
2. Transfer the onion and garlic to a medium bowl, and allow to cool slightly. To the onion mixture, add the bread crumbs, mustard, and parsley, and stir until well mixed. Add the ground turkey, and gently mix with your hands or a fork until just combined. 
3. Form 4 patties, slightly larger in diameter than the hamburger buns, as the patties will shrink as they cook. Arrange the patties on a plate using parchment paper to separate the patties. Refrigerate them until you are ready to grill them.
4. Heat a grill or grill pan to medium-high heat. Clean and lightly oil the hot grill. Season the patties with salt and pepper, then brush with oil. Grill the patties, covered, until they are cooked through, about 4 to 6 minutes per side. During the last minute of cooking, top each burger with cheese, if using, and cover the grill until the cheese is melted. 
5. Serve the burgers on buns with desired toppings.

MAKE IT AHEAD:

Make the uncooked turkey burger patties up to two days in advance, and store them in the refrigerator in an airtight container.

Reprinted with persmission from So Much To Celebrate: Entertaining the Ones You Love the Whole Year Through by Katie Jacobs, 2018, Thomoas Nelson. Find Katie at StylingMyEverday.com.