Heat 2 tablespoons of the olive oil in a large pot over medium-high heat. Add the beef cubes in a single layer and brown on all sides. Remove the beef to a plate.
Lower the heat to medium and add the remaining tablespoon of olive oil. Stir in the sliced leeks and minced garlic. Cook until the leeks soften.
Sprinkle the flour over the leeks and stir for one minute to form a light roux.
Return the beef to the pot. Add the beef broth, cubed celery root, thyme sprigs, bay leaf, and a pinch of salt and pepper. Stir well.
Bring the mixture to a simmer, then cover and cook on low for 1½ hours until the beef is tender.
Remove 1 cup of the softened celery root pieces. Mash them with a fork until smooth, then stir the mash back into the pot to thicken the broth.
Discard the thyme sprigs and bay leaf. Taste and adjust salt and pepper before serving.