01 - Combine cranberries, sugar, water, orange zest, and salt in a small saucepan. Simmer over medium heat, stirring occasionally, until cranberries burst and sauce thickens, about 8 to 10 minutes. Remove from heat and let cool slightly.
02 - In a mixing bowl, combine ground turkey, finely chopped onion, minced garlic, chopped parsley, Dijon mustard, salt, and black pepper. Mix gently until just combined. Shape mixture into four equal-sized patties.
03 - Heat olive oil in a skillet over medium heat. Cook turkey patties for 5 to 6 minutes per side until golden brown and fully cooked, reaching an internal temperature of 165°F (74°C). Transfer patties to a plate.
04 - If desired, melt butter in the skillet over medium heat and toast the buns cut side down until golden brown.
05 - Place lettuce leaves on the bottom half of each bun. Top with a turkey patty, sliced Brie, a generous spoonful of cranberry sauce, and optional sliced red onion. Cover with the top bun.
06 - Serve immediately while warm and cheese is melted.