Start with baking the beetroots:
Cut any leaves from the beetroots and wash the bulbs.
Place them in a row lengthways in the middle of two large sheets (one on top of another) of an aluminium foil. Drizzle the beetroots with some olive oil and fold the aluminium foil to bake en Papillote. To fold the foil en Papillote start with the long borders of the foil. Join them together above the beetroots and fold tightly. Next fold the ends.
Bake in the preheated oven at 160 C for about 80 minutes.
Take the beetroots from the oven, discard the aluminium foil (be careful as it might be still hot) and leave to cool.
Now move to the blind baking of the puff pastry.
Roll out the puff pastry (I used a shop bought one) onto the baking parchment and fit it into a round baking tray of 26cm in diameter. There should be some extra pastry around the margins of the tray that is best to fold gently, creating a nice little edge.
Prick the pastry with a fork and bake in the preheated oven at 180 C (temperature for a non fan assisted oven, otherwise set the temperature to 170 C) for around 15 minutes. The pastry should become light golden in colour. You could weigh the pastry down with some dry beans or ceramic balls but it is not necessary here. The tart will still be wafer thin and crunchy.
Set aside and wait until it cools down.
Once the beetroots are cold enough to manage, peel them and thinly slice with a help of a mandolin.
Scatter some thyme leaves on top of the pastry and tightly arrange the beetroot slices starting with the outer circle pressing them into the edges.
Brush the layered slices with the honey. Season with salt and a bit of pepper. Scatter some more thyme leaves and brush everything lightly with some olive oil.
Bake in the preheated oven to 180 C for 40 minutes. If in the meantime you will notice that the beetroot slices seem dry, coat them with another layer of olive oil and carry on baking.
Once the tart has baked, cool it down for a few minutes. Scatter over the walnuts, dot with crumbled or diced cheese and the herbs. Sprinkle some more salt and black pepper and drizzle more honey if you like.
Serve while still warm.
You could also bake your tart already with a cheese layer.
Just dot evenly the cheese of your choice on top of the pastry as the first layer. Scatter the walnuts and then arrange sliced beetroot on top and then bake for 40 minutes.