Preheat your oven to 400°F (200°C) and prepare a large rimmed baking sheet.
Peel and chop the carrots and parsnips into uniform bite-sized pieces, then toss them on the baking sheet with 0.5 tbsp of olive oil and half of the sea salt and black pepper.
Roast the root vegetables for 20-25 minutes, tossing halfway through, until they are tender and lightly caramelized.
While the vegetables roast, pat the venison loin dry with paper towels and coat all sides evenly with the Dijon mustard.
In a small bowl, combine the dried rosemary, dried thyme, garlic powder, and the remaining salt and pepper.
Press the herb mixture firmly onto the mustard-coated venison to create an even crust.
Heat the remaining 0.5 tbsp of olive oil and the ghee in an oven-safe skillet over medium-high heat until shimmering.
Sear the venison loin for 2 minutes per side until a deep golden-brown crust forms on all surfaces.
Transfer the skillet to the oven and roast for 5-8 minutes, or until the internal temperature reaches 130°F for medium-rare.
Remove the venison from the pan and let it rest on a cutting board for 5 minutes before slicing against the grain and serving with the roasted vegetables.