Preheat oven to 200C/Gas 6. Spray a large pan with oil and brown the beef. Add diced onion, carrots and celery, and allow to soften for 10 min.
Stir in chopped tomatoes, tomato purée, Worcestershire sauce, bay leaf, thyme, salt and pepper, water and Oxo cubes. Bring to the boil, cover and simmer for 30 min, stirring occasionally.
Meanwhile, boil celeriac until very tender, drain and mash with crème fraîche until as smooth as you wish. Heat oil in a pan and gently sauté leeks, then add them to the celeriac mash; season well.
Pour beef into a shallow, ovenproof dish and top with celeriac mix. Bake for 20-30 min, until top is golden brown.