Preheat oven to 425° and grease a roasting pan with some cooking oil (or you can use a large skillet).
Core and chop one of the apples, peel and quarter the onions, and cut celery ribs into pieces as well. Spread chopped fruit and veggies in the bottom of the roasting pan close together. The goal is to create a bed for the pork loin to sit on.
If you have a fat cap with is pretty good thickness, it's good to score the top so that the seasoning and wet rub flavor get through to the meat.
To score the top of the pork loin, use a sharp knife to make several slices through the fat cap, just to the meat, first in one direction and then in another, creating a diamond pattern on top.
Whisk Dijon mustard, maple syrup, dried minced garlic, dried minced onion, salt and pepper together and rub it generously, all over pork.
Place pork loin on top of fruit and veggies in the roasting pan.
Cooking the Pork:
Insert a digital instant read meat thermometer probe through the side of the pork, towards the center to track the internal temperature.
Place the roasting pan into the oven and cook at 425° for 15 minutes and then lower the temperature to 350°. Cook until the internal temperature reaches 145° degrees. Pork loin will take about 60-80 minutes to cook all together but tracking the internal temperature is the best way to tell if the pork is done.
Take the pork out of the oven and out of the roasting pan right away. Place it into a serving dish, cover it loosely with a sheet of foil and let it rest for about 15-20 minutes before cutting.
Optional Pan Drippings Gravy:
Strain the pan drippings (and the cooked veggies) from the roasting pan into a measuring cup through a fine mesh strainer. Press a few times on the veggies in the strainer to get the juices out. Discard what is left in the strainer. Note: there will be more juices from pork in the dish while it is resting, be sure to get those juices and add it to the pan drippings.
Measure and prepare all the ingredients to be ready on hand before starting to cook because this gravy will go fast.
Melt butter in a cooking pan over medium heat. Once melted, whisk in flour until smooth and blended.
While still whisking, start pouring in the pan drippings and chicken stock. Keep whisking until smooth. Taste to see if you need to add any salt and pepper.
Slowly stir as the gravy comes to low simmer and thickens. Cook for a couple extra minutes, stirring slowly, and take off heat.
Notes
*Gluten Free – use gluten free all purpose flour of needed instead of regular flour if you need to make a gluten free gravy. Remaining ingredients should already be gluten free but double check the packaging.