Preheat oven to 350°. If you wish, cover the roasting pan with foil.
Peel and quarter the onions. Cut celery ribs in fours as well. Cut 3 garlic cloves in half, length-wise. Place veggies in the middle of the roasting pan in one single layer, about the same size as the pork loin.
In a small bowl, combine dry herbs, salt, and pepper and mix.
Using a small knife, pierce holes in meat in several places over the top. Make holes that are about 1-inch deep. Push garlic into each hole. If garlic cloves are too big, cut them in half length-wise.
Rub pork loin with some olive oil and then herb mixture all over, even on the bottom.
Place pork fatty side up in the prepared roasting pan, on top of the veggies.
Bake until internal temperature reaches 145°. For this 4 pound pork, it will take about 1 hour and 20 minutes. Estimate about 20 minutes per pound but still check internal temperature.
Once pork reaches the temperature, take it out and tent it with foil. Let it rest for 15-20 minutes before cutting.
*Note: You can use the accumulated juices from roasting pork to make gravy. If not enough juices accumulated, add a little veggie broth.