Preheat the Traeger Grillor your oven to 350ºF, be sure the skillet you are baking the frittata in is well seasoned. Grease with additional ghee or olive oil if necessary.
In another skillet on the stovetop over medium-high heat, add the pancetta, cook for a few minutes, stirring frequently, then reduce the heat to medium and add the onions and mushrooms. Sauté another 8-10 minutes until the onions are translucent and tender. Add the garlic and season with a little salt and pepper. Stir well, give it another minute or two. Remove the pan from the heat and allow to cool. (You don't want to add hot veggies to the eggs)
Meanwhile, in a large bowl, add the eggs, milk, Chinese five spice (or roughly chopped thyme - see notes above), salt and pepper. Whisk until well combined.
Add the thinly sliced apples, add all the cheese minus 1/4 cup or so, and the greens. Using a spatula or wooden spoon, stir until well mixed.
Once the pancetta, onion, mushroom mixture is not longer hot and the grill (or oven) is up to temperature - add the cooked veggies to the egg mixture and stir well to combine.
Pour the egg mixture into the seasoned/prepared skillet.
Crumble the remaining cheese evenly over top.
Bake the frittata at 350ºF for 20-30 minutes until golden and totally set. I recommend you begin checking it at 15 minutes and continue checking every 5 minutes or so. Depending on the thickness of the frittata, it could take more or less time.
Let cool about 5 minutes before cutting.
Serve with fresh herbs or greens on top or a mixed greens salad on the side.