Cut the onion, carrots, and celeriac into 1 cm cubes. Cut the leek into 1 cm thick rings.
Heat the olive oil in a large casserole over medium heat. Add the onion, carrots, celeriac, and leek. Fry for 2 minutes.
Stir in the tomato paste and roast over low heat for 3-4 minutes.
Add the red wine and let it boil down. Add the remaining red wine and let it boil down again.
Add the broth, garlic cloves, bay leaf, and thyme sprigs. Bring to a boil.
Add the pork cheeks, reduce heat to low, cover with the lid, and simmer for about 1 hour until the meat is tender and reaches an internal temperature of 63 °C / 145 °F.
Remove the pork cheeks and keep warm. Strain the sauce through a fine sieve.
Reduce the sauce over medium heat until it reaches the desired consistency. Season with salt and pepper.