Lightly flour and season the oxtail pieces. In a large casserole dish, on a high heat, add a dash of oil and brown the floured oxtail.(You may need to do it in batches and be patient to get a nice brown).Remove once browned and set aside.
Now, in the same dish, reduce the heat slightly to medium and add a dash of fresh olive oil, then add the vegetables and the thyme to the pan. Cook until soft and then stir through the tomato purée and another tablespoon of flour.Cook for 5 mins, then add the wine and allow to reduce by half. Finally, add the beef stock and the browned oxtail pieces. Bring to a gentle simmer, cover and cook until very tender - approx 3-4 hrs
Once braised, remove the meat from the liquid and allow to cool. Pass the liquid through a fine sieve and then return to a pan over a high heat and reduce until you have a gravy-like consistency
Pick the meat from the bones and put the meat back in with the reduced sauce. Season to taste. (You will reheat this before serving).
Celeriac 'pasta sheets'
Peel and thinly slice half of the celeriac into 12 thin sheets. Trim the sheets into large circles and blanch in boiling, salted water for 2 mins. Refresh in iced water, then drain on kitchen paper.
Celeriac puree
Dice the remaining celeriac and boil in salted water until tender. Place in a food processor with the cream and blend into a smooth purée. Season to taste and keep warm or set aside to reheat before serving.
Horseradish
In a bowl, mix the cream with the grated horseradish and add a pinch of salt and sugar. Use a hand whisk to whip until thick
Spinach
In a pan over a medium heat, sauté the spinach in the butter until tender, then season and keep warm.
Building your lasagne
Gently reheat the celeriac sheets in a little simmering water and make sure all the other elements are reheated (apart from the horseradish cream)In a small bowl or plate, layer up the spinach, celeriac discs, celeriac purée and braised oxtail as if making a lasagne. Top with a spoon of horseradish cream and a few chopped chives. Serve immediately.