Caramelize 2 chopped yellow onions in the melted butter/olive oil with a pinch of salt , sauteeing on medium until jammy and browned (about 20 minutes).
Finely grate the portobello mushrooms using a box grater.
In a large bowl, gently combine the turkey, mushrooms, shredded cheese, garlic powder, salt, pepper, and parsley. Mix just until combined—don’t overmix.
Form into 6 large patties, pressing a slight indentation into the center of each.
Cook on a lightly oiled grill, or skillet over medium-high heat for about 5–6 minutes per side, or until the internal temperature reaches 165°F.
Let the burgers rest for 3 minutes before serving.
