Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Peel and chop the carrots and parsnips into 1-inch chunks, and slice the red onion into wedges.
Toss the vegetables on the baking sheet with 0.5 tablespoon of olive oil and half of the sea salt and black pepper.
Roast the vegetables for 25 to 30 minutes, tossing halfway through, until they are tender and caramelized on the edges.
While the vegetables roast, season the venison loin with the remaining sea salt and black pepper.
Heat the remaining 0.5 tablespoon of olive oil in a cast-iron skillet over medium-high heat.
Sear the venison for 3 to 4 minutes per side for medium-rare, adding the smashed garlic clove, rosemary, and thyme to the pan in the last 2 minutes to baste the meat.
Remove the venison from the pan and let it rest for 5 minutes before slicing against the grain.
Serve the sliced venison alongside the roasted root vegetables, drizzling any remaining pan juices over the top.