Cut the leaves from the tops of the beetroot and set aside.
Half-fill a saucepan with water, then add the whole beetroot and bring the water to the boil. Reduce the heat until the water is simmering, then simmer for 30-45 minutes, or until the beetroot are tender when pierced with the point of a knife. Drain well and set aside to cool.
Meanwhile, bring the buffalo milk to the boil in a separate saucepan, without stirring. When the milk is just coming to the boil, whisk in the lemon juice – you will immediately see a reaction as the milk solidifies and separates into curds and whey.
Carefully strain the buffalo milk curds and whey through a sieve suspended over a large bowl. Set aside to drain for 5 minutes - the longer you leave it to strain, the more solid a cheese it will become. Once strained, chill the solid curd cheese in the fridge until needed (discard the liquid whey).
Heat half the olive oil in a frying pan over a medium heat. When the oil is hot, add the bread cubes, rosemary and thyme leaves and fry, stirring well, until the bread is golden-brown and crisp. Transfer the pan contents to a bowl, then drizzle over the remaining olive oil and all of the sherry vinegar.
Add the cumin seeds and reserved beetroot leaves to the pan used to cook the bread and stir-fry until the beetroot leaves have just wilted. Add the pan contents to the bowl with the bread cubes.
When the beetroot are cool enough to handle, cut them into chunks and add them to the bowl containing the bread cubes. Mix in the remaining herbs and season, to taste, with salt and freshly ground black pepper.
To serve, divide the salad among four serving plates. Top with spoonfuls of the strained buffalo curd cheese. Drizzle over a little olive oil, then garnish with the oregano sprigs, oilseed rape, if using, and dill sprigs.