Heat the oven to 220C/fan 200C/gas 7. Fry the onion and cumin seeds in a spray of olive oil for 5 minutes, until soft. Tip into a blender with the beetroot, chickpeas, egg and half the sesame seeds. Blend until combined and season well. Shape into about 8 patties. Put onto a baking tray lined with baking paper. (The mix will be a little wetter than expected, but this is what you want.) Scatter over the remaining sesame seeds. Bake for 30-40 minutes until firm, and allow to cool for 5 minutes. Mash the feta with the herbs and yogurt to make a smooth-ish sauce and season.