Soak 40 g (1½ oz/¼ cup) raw cashew nuts in water for at least 4 hours. Drain and rinse. Blitz in a food processor with 3 tbsp nutritional yeast, ½ tsp celery salt, ½ tsp sea salt flakes and a pinch each of white pepper and mustard powder. In a pan, whisk 200 ml (7 fl oz) unsweetened soya milk with 4 tsp agar-agar powder and 1 tbsp olive oil. Cook for 5 minutes until thick, then add to the nut mixture with 4 tbsp lemon juice. Blitz. Pour into an oiled ramekin, smooth the top and cover. Chill until set, then invert onto a plate.