Lightly toast the cumin in a dry pan, then tip into a bowl.Add the flour, baking powder and seasoning with 250g of the yoghurt and mix together until you have a rough dough.Turn out onto a lightly floured surface and knead until the dough just comes together. Place in a lightly greased bowl, cover with a damp tea towel and put to one side.Peel, destone and chop the avocado into chunks, then place in a bowl. Crumble in the feta. Add a drizzle of oil and season to taste.In another bowl, stir the harissa into the rest of the yoghurt.Divide the dough into eight balls. Roll each one on a lightly floured surface into an oval shape, roughly 3mm thick.Place a griddle pan over a high heat. Once hot. griddle each flatbread for 2 to 3 minutes, until puffed up and charred, turning halfway.Off the heat, brush the flatbread with a little oil, then serve with the avocado salad and harissa yoghurt.