Wash and dry all produce. Adjust rack to middle position and preheat oven to 400 degrees. Peel carrots and parsnips. Slice both into 1/4-inch-thick rounds on a slight diagonal. Halve and peel onion, then cut into 1/2-inch wedges. Strip leaves from thyme and discard stems; roughly chop leaves. Mince or grate garlic.

Toss together carrots, parsnips, onion, half the thyme, 1 TBSP olive oil, and a pinch of salt and pepper on a baking sheet. Roast in oven until tender and lightly browned, 23-25 minutes, tossing halfway through.

Heat a drizzle of oil in a large pan over medium-high heat. Pat pork dry with a paper towel. Season all over with salt and pepper. Add to pan and cook until browned and nearly cooked through, 3-4 minutes per side. Remove from pan and set aside.

Heat a drizzle of oil in same pan over medium heat. Add garlic and toss until fragrant, about 30 seconds. Stir in a pinch of cayenne (to taste—it’s spicy), 1 TBSP sugar, remaining thyme, stock concentrate, and 1/2 cup water, scraping up any browned bits on bottom. Bring to a simmer and let bubble until reduced by half, 2-3 minutes. Season with salt and pepper.

Return pork to pan and turn to coat in glaze. Cook until pork reaches desired doneness, 1-2 minutes longer. Using tongs, pick up each piece and gently shake above pan to let excess glaze drip off, then transfer to a cutting board.

Thinly slice pork. Divide veggies and pork between plates, then drizzle with any remaining glaze.