Melt butter, maple syrup, vinegar, and Dijon together in a medium pan and bring to a simmer over medium-high heat. Reduce heat to medium-low and allow to simmer 3-5 minutes until sauce is reduced, stirring occasionally. Divide maple glaze between 2 small bowls with 1/4 cup glaze in one and the rest in the other.
Preheat oven to 375F and grease a large sheet pan. Combine onion, beets, parsnips, turnips, carrots, garlic, and 2 tablespoons olive oil and arrange in a single layer on sheet pan, leaving a space in the center for pork loin. Season vegetables with salt and pepper to taste.
Season pork loin well all over with salt and pepper to taste. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add pork loin fat-side down and sear on all sides until golden-brown. Transfer pork loin to sheet pan with vegetables.
Brush pork loin all over with 1/4 cup maple glaze (save the rest for glazing later) and sprinkle pork and vegetables with rosemary. Roast pork loin and vegetables at 375F 30-40 minutes until pork registers 135-140F and vegetables are tender.
Remove pork from oven, tent with tin foil, and allow to rest 20 minutes.
Transfer pork loin to a cutting board and cut into slices. Reheat reserved glaze over low heat until warm and serve drizzled over pork loin slices with root vegetables. Enjoy!