Put three medium-large beetroot in a saucepan with around four or five pickling onions, and boil til the beetroot are just soft (this will take quite a long time). Whilst waiting for the beetroot to cook make pastry: combine 300g of plain flour with 100-150g butter in food processor to a “crumb”, then add small amounts of chilled water until the pastry comes together. If you have time, roll into a ball and chill.
Cut woody top out of beetroot, and peel; also remove skin from onions and roughly chop. Cut beetroot in half lengthways (this gives a flat surface to sit beetroot on board) then cut into half centimetre semi-circle slices. Arrange these on baking paper in a baking tray so they overlap slightly as pictured. Top with the chopped boiled onion, then roll out the pastry and tuck into the backing paper like a blanket. Brush with a beaten egg and bake at 200C for about 20 minutes until the pastry is golden brown and well-cooked.
Allow to cool briefly, then place a board on top of the tray, flip the tart onto the board, and top with goats cheese. (I used a dill-flavoured cheese, and also topped with mixed Asian mushrooms cooked in a generous quantity of truffle butter, which is optional!)