Preheat Traeger Grill to at least 375°. Place cast iron pan in grill while it heats.
While the grill heats, pat dry the beef pieces with a paper towel and lightly season.
When the pan is very hot, cook the beef in batches until browned on all sides. Transfer to a bowl and cook remaining beef.
Add a bit of butter to the empty pan and cook the onion, carrot and celery until crisp-tender.
Stir in the remaining butter to melt, then whisk in the flour to create a roux. Cook until well combined and browned.
Stir in beef broth and wine. Then add tomato paste and garlic. Stir vigorously to combine.
Add the par-cooked beef back to the pan and add the frozen peas. Stir to combine and then close the grill lid, allowing to simmer for about 15 minutes until slightly thickened.
When slightly thickened, bring pan inside and top with prepared mashed potatoes (remember to add 1/2 the cheese to the potatoes). Spread evenly and top with remaining cheese.
Return pan to grill and cook for 20 minutes until golden on top and bubbling.
Carefully remove from grill, allow to sit for 5 minutes, and serve.