Cut a pocket between the fat and meat of the loin. Reserve the outside leaves of the leek. Chop the remainder and fry until softened in butter.
Mix the cheese, bread crumbs, half the mustard, seasoning and leeks together. Place in the pocket, and secure joint with string.
Mix the softened butter, remaining mustard, thyme and sage and rub into the pork. Pierce the loin and insert garlic slices.
Place on a trivet in a roasting pan and cook in a preheated oven at 425 degrees F for 15 minutes, then at 350 degrees for 1 to 1 1/4 hours.
Place the reserved leek leaves in a bowl of boiling water to soften for 2 to 3 minutes.
Boil carrots and celery root until just tender.
Drain the leeks and cut into narrow strips. Make bundles of carrots and celery root and tie with the leek strips.
Remove the string from the roast and place on a warm dish.