STEP 1: Preheat the oven to 200c. Wash, scrub then dry beetroots (no need to peel). Place beets and onions in a roasting tin tossed with 2 Tbsp of olive oil, season with a generous pinch of sea salt.
Roast for 35 minutes then remove from the oven until cool enough to handle. Cut off the top and bottom of the beetroots, using a piece of kitchen paper, gently rub the skin to remove (it should come off easily). Slice beetroots into disks, no more than 1/2 cm thick.
STEP 2: Roll out the pastry onto a parchment lined baking sheet. Score the edges all the way around leaving a 2cm border. Brush the sheet with a little olive oil. Prick the base of the pastry all over with a fork before baking for 10-15 minutes until golden. Once cool, press the centre of the pastry down if it has risen too much.
STEP 3: Dice the mushrooms then fry in the olive oil, seasoned with a little salt, until deeply browned. This should take between 10 and 15 minutes, do not be tempted to stir the mushrooms too often. Simply let them cook, allowing their moisture to release and then evaporate in the hot pan stirring only a couple of times to turn. Remove from the heat and allow to cool.
STEP 4: Squeeze the onions from their skin, they should slide out in gloriously slippery, sweet slithers. Mix with the mushrooms and season with pepper to suit your taste.
STEP 5: Using either an electric hand whisk, or a stand mixer, beat the Crematta, Olive oil and cornflour until completely combined. Transfer to a small bowl then cover and chill in the freezer for 15 minutes.
STEP 6: When ready to assemble the tart, spread the Crematta over the base then spoon over the mushrooms and onion mixture. Arrange the beetroot over the top before drizzling with the lemon herb vinaigrette. Slice and serve.