Preheat the oven to 200℃/180℃ fan/Gas 6. Brush the beetroot with a little olive oil and then wrap each one seperately in foil and roast for 1 hour. Leave to cool completely, then peel (the skin should come off easily) and grate coarsely.
Place the spinach, a pinch of salt and 3 tablespoons of water in a saucepan, cover with a lid and cook on the lowest possible heat for 10 minutes until the spinach has wilted and turned a darker shade of green (stirring halfway through and adding a little more water if needed). Give it a good stir, turn the heat to medium and cook for 1-2 minutes or until all the water has evaporated. Set aside to cool before chopping roughly.
Place the grated beetroot, spinach, yoghurt, lime juice, pepper and a pinch of salt in a large bowl. Stir gently until they are completely mixed. Taste and add more salt if needed.
Place in the fridge for an hour; this will help blend the yoghurt and beetroot flavour and colour. Give it a good stir and garnish with sesame seeds before serving.