Preheat oven to 350 degrees F.
Season the pork loin with salt, black pepper, paprika, and caraway seeds.
Spread the mustard over pork.
Heat the vegetable oil in a large skillet over medium-high heat.
Fry the pork until browned on all sides, about 15 minutes.
Transfer the pork to a large roasting pan.
Add the onion, carrots, celeriac, and spring onion to the same skillet used to fry the pork
Cook and stir until the vegetables are tender, about 10 minutes.
Spread vegetable mixture around pork in roasting pan.
Pour 1 cup of 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.
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, add butter and cornstarch and simmer until sauce is thickened, about 5 minutes.
Slice pork and serve with sauce and vegetables.