Rub roast with seasoned salt and let sit for 30 minutes.

Meanwhile, mix olive oil, black pepper, and rosemary together in a bowl. Rub roast with mixture. Let sit for 1 hour.

Heat a skillet over medium-high heat. Brown roast on all sides in the hot skillet, 10 to 15 minutes.

Preheat the oven to 350 degrees F (175 degrees C).

Place celery, carrots, and onion in a large, shallow baking pan and cover with water. Place roast on top, bone side-down.

Bake in the preheated oven until slightly pink in the center, about 2 hours, or to desired doneness. Remove roast from the pan, cover, and let rest for 30 minutes. Remove vegetables from the pan.

Place the pan on the stovetop over medium to high heat to make the gravy and bring to a simmer. Stir in milk. Sprinkle in flour for desired thickness and cook until gravy has thickened. Serve with roast.