FOR THE GRATIN
250ml oat cream
500ml non-dairy milk
4 sprigs of thyme
2 sprigs of rosemary
½ nutmeg, finely grated
2 bay leaves
50g non-dairy butter
1 tbsp wholegrain mustard
1 large celeriac, peeled and cut into 5mm-thick slices
3 large floury potatoes, such as King Edward or Maris Piper, peeled and cut into 5mm-thick slices
Sea salt and freshly milled black pepper
FOR THE SOFRITTO
2 tbsp vegetable oil
1 onion, peeled and finely diced
1 leek, white part only, finely diced
1 celery stick, finely diced
2 garlic cloves, peeled and finely grated
1 tbsp tomato purée
½ tsp smoked paprika
100ml red wine
100 stoned Kalamata olives, finely chopped
25ml olive oil
FOR THE BUTTON ONIONS
2 tbsp non-dairy butter
200g pearl onions, peeled but left whole
1 tbsp balsamic vinegar
Start by making the gratin. Put the cream, milk, thyme, rosemary, cloves, grated nutmeg and bay leaves into a large saucepan over a moderate heat. Bring to a gentle simmer for 15 minutes. Remove from the heat, cover and set aside for 15 minutes to allow the milk to infuse. Strain off the milk and whisk in the butter, mustard and seasoning.
Preheat the oven to 170°C/fan 150°C/gas mark 3.
From a layer of overlapping celeriac slices in the base of an ovenproof gratin dish, about 20 x 20cm. Spoon over a little of the sauce, then repeat the process with a layer of potato slices. Continue until all the vegetables have been layered. Pour over any remaining sauce and finish with a good grinding of black pepper.
Place on a baking tray to catch any sauce that bubbles over and cook in the oven for 1-1½ hours until golden and a knife can be inserted with no resistance.
Start the sofritto 1 hour into the gratin cooking. Heat the vegetable oil in a large frying pan over a moderately high heat. When hot, add the onion, leek, celery and garlic and season well. Cook for 10 minutes or until soft. Stir in the tomato purée and paprika and mix well. Add the red wine and cook for 10 minutes until almost evaporated. Remove from the heat, add the olives and olive oil and mix well.
For the button onions, heat the butter in a medium frying pan until hot. Add the onions, season well and cook until brown all over. Add the vinegar and cook for a further 5 minutes until sticky.
Serve the gratin with the sofritto and onions on the side, with your favourite green vegetables.
Sofritto is essentially a base sauce in many Latin cuisines. It always includes cooked onions, but the rest of the ingredients are wide ranging, depending on the origin of the cuisine. I have used red wine, tomatoes and olives in my version, which add a piquant complement to the gratin.