How to Make It
Preheat broiler to HIGH, and position middle oven rack 6 inches from heat. Rub pork tenderloin with 1 tablespoon of the olive oil, and season on all sides with thyme, 1 teaspoon of the salt, and 1/2 teaspoon of the pepper. Place on a rimmed baking sheet, and broil in preheated oven until nicely browned, about 5 minutes on each side. Remove from oven, and reduce oven temperature to 450°F.
Toss together broccoli, squash, onion wedges, and remaining 1 1/2 tablespoons olive oil, 1 1/2 teaspoons salt, and 1/2 teaspoon pepper in a medium bowl. Move pork to 1 end of the baking sheet; spread vegetables and lemon halves in a single layer on other end of baking sheet. Bake at 450°F until a thermometer inserted in thickest part of pork registers 140°F, 15 to 20 minutes.
Transfer pork to a cutting board, and let rest 5 minutes. Meanwhile, add butter to baking sheet, and toss with vegetables until butter melts. Squeeze baked lemon over vegetables, and stir. Serve immediately.