Preheat oven to 425°F and grease a roasting pan with some cooking oil (or use a large skillet).

Core and chop one of the apples, peel and quarter the onions, and cut celery ribs into pieces. Spread chopped fruit and veggies in the bottom of the roasting pan close together.

Score the top of the pork loin with slices through the fat cap to allow seasoning to penetrate.

Whisk together Dijon mustard, maple syrup, dried minced garlic, dried minced onion, salt, and pepper. Rub this mixture generously all over the pork loin.

Place pork loin on top of the fruit and veggies in the roasting pan.

Insert a digital instant-read meat thermometer probe into the pork towards the center.

Cook in the preheated oven for 15 minutes at 425°F, then lower the temperature to 350°F. Cook until the internal temperature reaches 145°F. Total cooking time will be about 60-80 minutes.

Remove the pork from the oven and the roasting pan. Place it into a serving dish, cover loosely with foil, and let it rest for 15-20 minutes before cutting.

Strain the pan drippings and vegetables through a fine mesh strainer.

Melt butter in a cooking pan over medium heat. Whisk in flour until smooth.

Pour in the pan drippings and chicken stock while whisking until smooth. Adjust seasoning with salt and pepper.

Simmer and stir until the gravy thickens, then remove from heat.