If you're using a conventional oven, preheat it to 325°F. An air-fryer oven doesn't need preheating. (A basket-style air fryer won't fit the pie pan — use a conventional oven.)
Cook the diced onion in 1 Tbsp butter with a dash of salt until soft. Add the garlic and cook until fragrant.
Remove the onion and garlic from the pan. In the same pan, cook the ground turkey until the juices run clear.
Take the pan off the heat, return the onion and garlic, and let it all cool. (This matters — pour egg over hot meat and it starts to cook on contact, and you lose the smooth set.)
While it cools, butter the glass pie pan, bottom and sides.
Crack the 6 eggs into a mixing bowl and beat with the ½ cup half & half.
Once the meat mixture is fully cooled, spread it into the buttered pan. Scatter the grated cheese over it, then pour the egg mixture in.
Bake at 325°F for 40–45 minutes, until fully set and no longer jiggly.
Cool at least 10 minutes before slicing, then cut into 6. Refrigerate in airtight containers.