Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Peel and chop the carrots, parsnips, and sweet potato into uniform 1-inch cubes.
Toss the vegetables on the baking sheet with half of the avocado oil, half of the sea salt, and half of the black pepper.
Roast the vegetables for 25-30 minutes, tossing halfway through, until they are tender and golden brown.
While the vegetables roast, pat the venison loin dry with paper towels and season all sides with the remaining salt and pepper.
Heat the remaining avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Place the venison in the pan and sear for 3-4 minutes per side for medium-rare, or until it reaches an internal temperature of 130-135°F.
During the last minute of cooking, add the minced garlic and rosemary to the pan, spooning the flavored oil over the meat.
Remove the venison from the pan and let it rest on a cutting board for at least 5 minutes before slicing against the grain.
Serve the sliced venison alongside the roasted root vegetables.