Preheat the oven to 200C/400F/gas 6. Use a little of the butter to grease the sides of a large heatproof casserole. Peel all the vegetables except the onion and shred them, using the shredding blade of the food processor. Turn them all out on a large clean tea towel and pat and squeeze as dry as possible. Put the casserole over a moderate heat on the hob and melt the remaining butter in it. Peel and chop the onion and garlic, then saute them briefly until just translucent. Add the shredded vegetables and turn for a minute or so to coat with butter. Remove from the heat. Grate the cheese and mix half into the cream, then season it with the paprika, mustard, salt and pepper. Pour the cream over the shredded vegetables and sprinkle the remaining cheese over the top. Bake for about 30 minutes, until golden brown.