Preheat oven to 375°F.
Heat a cast-iron skillet over medium heat. Add olive oil and butter.
Add potatoes, celery, carrots, onion, and garlic. Sauté 8–10 minutes until vegetables begin to soften.
Season with fennel, onion powder, garlic powder, thyme, and salt. Stir well.
Add ground venison and cook 6–8 minutes, breaking it up, until browned and just cooked through.
Transfer mixture to a pie plate or shallow baking dish.
Sprinkle evenly with shredded cheddar cheese.
Lay puff pastry over the top, trim edges, and cut small slits for steam.
Brush pastry with egg wash.
Bake uncovered for 35–40 minutes, until pastry is golden and flaky.
Rest 5 minutes before serving.