Pat the 8 veal cheeks dry and season with salt and pepper from the grinder.
Heat 2 tablespoons olive oil in a large roasting pan over medium-high heat.
Sear the veal cheeks until golden brown on both sides, about 4-5 minutes per side. Remove and set aside.
Cut the onions, carrots, and celeriac into large cubes.
In the same pan over medium-high heat, roast the vegetable cubes with 1 tablespoon tomato paste, stirring occasionally, for about 3-5 minutes.
Deglaze the pan with 400 ml dry red wine and 1 tablespoon sugar beet syrup, scraping up browned bits. Boil until reduced by about half, roughly 5-8 minutes.
Wash and dry 3 sprigs thyme and 3 sprigs rosemary. Add them to the pan along with 1 bay leaf and 8 juniper berries.
Pour in 500 ml veal or beef stock. Return the seared veal cheeks to the pan.
Cover and braise in the oven at 160°C / 320°F (convection, on the 2nd or 3rd rack from the bottom) for 2 to 2.5 hours. Every 30 minutes, baste the cheeks with some of the braising liquid.
Transfer the veal cheeks to a warm plate. Pour the braising liquid through a sieve into a saucepan, pressing the vegetables gently to extract liquid.
Over medium-high heat, reduce the sauce by about half, roughly 10-15 minutes.
Season to taste with salt and pepper from the grinder. Stir in 1 tablespoon Dijon mustard. Do not boil after adding mustard.
For best flavor, prepare this dish up to 1 day ahead; cool completely and refrigerate. The veal cheeks will rest and absorb more sauce flavor. They are typically very tender after 2.5 hours of braising.