Preheat the oven to 220 °C. Line a baking sheet with parchment.
Add plenty of salt to an oven tray and place the beetroots on top. The beetroots should be whole and unpeeled. I use 30–40 g coarse salt, but regular is fine and the amount isn't very important – just be generous. Bake in the middle of the oven until you can easily pierce through with a sharp knife, anything from 45 minutes to a couple of hours, depending on the size of your beetroots. Set aside to cool.
Mix together all the ingredients for the marinade. Season to taste with salt and pepper. Peel and chop the beetroots into bite size chunks and add to the marinade (discard the peel and salt). Mix and leave at room temperature for half an hour.
Meanwhile, mix the yoghurt with the crumbled feta cheese. I like it chunky and make do with a fork or spoon for this, but use a mixer if you prefer it completely smooth.
Heat a small, thick bottomed frying pan over medium heat. Toast the pistachios in a dry pan for a few minutes, shaking the pan regularly to avoid burning. You could also use the oven while the beetroots are roasting, in which case 4–5 minutes should suffice. Cool and chop roughly.
When ready to eat, add most of the beetroot chunks and a little of the marinade to a large or two smaller serving plates. The chunks should fit nicely in one layer. Divide most of the feta cream in between. Add the remaining beets, the remaining feta cream and as much of the marinade as you like. Top with pistachios, dill and a sprinkle of sumac. Serve at once.