►Add olive oil and sliced onions to a medium skillet set over medium-high heat. Sprinkle salt over onions and stir. Cook for 5 minutes, stirring occasionally.

►Pour in vegetable stock and stir to combine. Reduce heat to low. Cook onions for another 20 to 25 minutes, stirring occasionally. Remove from heat and set aside to cool.

►Add ground turkey, egg, salt, black pepper, cumin, garlic powder, and melted butter to a large mixing bowl. Mix to combine with your hands or a wooden spoon.

►Shape turkey mixture into 4 1-inch-thick patties. Transfer patties to a small baking sheet or dinner plate, cover with plastic wrap, and chill in refrigerator for 10 minutes or up to overnight.

►Grease a 12-inch cast-iron skillet and heat over medium- high for 5 minutes.

►Add turkey patties to skillet and cook on one side for 5 minutes. Flip patties, top each with 2 slices of muenster, and cook for an additional 3 to 5 minutes.

►Serve turkey burgers on your preferred burger bun. Garnish with lettuce, avocado, tomato, and caramelized onions.