Use paper towels to pat the pork loin dry and set it aside.
In a large plastic zipper baggie, add 2 Tablespoons of the olive oil, salt, pepper, thyme, garlic, balsamic vinegar, and Dijon. Mix it together. Then add the pork roast and coat in the mixture. Set this aside to marinate for 30 minutes. If you marinate longer, put in the fridge. Allow to come to room temperature before cooking.
Preheat oven to 350°F.
Heat remaining 2 tablespoons of the olive oil in a large skillet over medium-high heat.
Add the roast to the heated skillet (save the marinade) and sear each side for 3-4 minutes. Remove from heat.
If using an oven safe skillet, add the onions to the pan along with the rosemary sprig (otherwise transfer all to a roasting pan) and pour the remaining marinade over it.
Place in the oven and roast for 45-50 minutes, or until the pork roast is cooked through with an internal temperature of 140° F.
Remove from the oven and allow the roast to rest, lightly covered, for 10 minutes before slicing and serving. Enjoy!
Notes
Store leftovers in an airtight container for up to 3-4 days in the refrigerator.