Start by preparing your beetroots; slice each one as thinly as you are able, 2–3mm thick. Over a low heat, melt the butter in a large frying pan and add the uncooked golden and candy beetroot slices.
Cut out a circle of non-stick baking paper, roughly the size of your frying pan. Scrunch it up and spread it out again into a crinkled paper circle (this helps trap steam to aid the cooking process). Lay it over your beetroot slices and allow to steam for 8 minutes or until just soft. Set aside to cool slightly.
Preheat the oven to 200°C/fan 180°C/gas mark 6. On a lightly floured work surface, roll out the pastry to a rectangle measuring roughly 20cm x 30cm, about 3mm thick. Transfer it to a baking sheet and score a 2cm border around the edge using a sharp knife, making sure not to cut through the pastry.
Place the mascarpone in a small bowl and mix to soften, then stir in the horseradish until evenly distributed. Season with a little salt and pepper. Spread the mascarpone mixture over the puff pastry, taking care not to go beyond the scored border. Top with a mixture of the softened beetroot and the ready-cooked beetroot slices. Brush each slice with olive oil, sprinkle over the thyme leaves, and bake for 30 minutes.
5Remove from the oven and allow to cool slightly, then brush with warm honey just before serving.