Heat broiler. On a cutting board, sprinkle a generous pinch of salt over garlic and drag the blade of a knife at an angle across mixture until a paste forms. Transfer garlic paste to a small bowl and whisk in 2 teaspoons oil. Place pork on a broilerproof rimmed baking sheet. Rub all over with garlic oil and season with salt and pepper. Broil until browned, about 10 minutes, flipping once. On another rimmed baking sheet, toss together brussels sprouts, parsnips, onions, thyme, and 2 tablespoons oil; season with salt and pepper.
Reduce heat to 450 degrees, with racks in upper and lower thirds. Roast vegetables on bottom rack and pork on top rack, about 10 minutes (a thermometer should read 145 degrees when inserted in center of pork). Remove pork from oven, transfer to a cutting board, and tent with foil. Transfer vegetables to top rack and roast until tender and browned in spots, 15 minutes. Remove from oven; toss with lemon juice. Refrigerate half of 1 tenderloin for another use. Thinly slice remaining pork; serve with vegetables.