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

Season pork loin with salt, black pepper, paprika, and caraway seeds; spread mustard over pork.

Heat vegetable oil in a large skillet over medium-high heat; fry pork until browned on all sides, about 15 minutes. Transfer pork to a large roasting pan. Add onion, carrots, celeriac, and spring onion to the same skillet used to fry pork; cook and stir until vegetables are tender, about 10 minutes. Spread vegetable mixture around pork in roasting pan.

Pour 1 cup hot water into the skillet used to cook the pork and vegetables; simmer and scrape up any browned bits from the bottom with a spatula. Pour water over pork.

Bake in preheated oven until pork is slightly pink in the center, 2 1/2 to 3 hours, basting often. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C). If vegetables become dry, add 1 cup hot water, or as needed.

Transfer pork roast to a large platter and keep warm. Strain remaining liquid through a sieve into a saucepan; reserve vegetables.

Bring liquid to a boil and add butter and cornstarch; simmer until sauce is thickened, about 5 minutes. Slice pork and serve with sauce and vegetables.