First, make the pastry by running the butter into the flour until the consistency of breadcrumbs before adding water (starting with two tablespoons) and the egg to bring it together to make pastry.
Wrap the pastry in a beeswax wrap or cling film and place in the fridge for half an hour.
Meanwhile, drizzle the beetroot with a tablespoon of olive oil, season and roast in a pre-heated oven at 180 degrees Celsius (350 degrees Fahrenheit) for 50 minutes.
Roll out the pastry on a floured surface and line a greased tart tin before lining and filling it with baking beads or rice and baking for 20 minutes.
Make the filling by gently cooking the onions in butter in a pan before adding the cooker potatoes. Mix the eggs with the cream and thyme leaves and season well before adding to the onions and potatoes.
Remove the beads or rice, brush the base with some of the egg and then bake again for five minutes.
Pour the filling into the tart case, place roasted beetroot chunks and goat’s cheese across the top of the tart and return to the oven for another 40 minutes.