Preheat the oven to 200°C/400ºF/gas 6. At the same time, put a roasting tray on the bottom shelf and boil a kettle of water. Line a baking sheet with baking parchment.
If using the butter and sesame seeds, generously butter the baking sheet instead of lining with paper, then sprinkle liberally with sesame or poppy seeds.
Separate the eggs and put the whites aside for later. Put the egg yolks, linseed, potato starch, ground almonds, yoghurt and baking powder into a large bowl and beat with a balloon whisk or electric whisk until the mixture is smooth, pale and creamy.
In a large, clean bowl, whisk the egg whites with 1 teaspoon of sea salt, using a balloon whisk, electric whisk or stand mixer, until stiff peaks form.
Stir a spoonful of the beaten egg white into the almond mixture to loosen it, then carefully fold in the rest using a spatula, without knocking out too much air.
Spoon the mixture onto the baking sheet and gently spread into an oval or teardrop shape, about 2cm thick. Sprinkle with your chosen seeds.
Put the baking sheet into the oven and pour boiling water into the roasting tin to half fill it. Bake the flatbread for 12 to 15 minutes, depending on thickness, until risen, golden and springy to the touch. This flatbread is best eaten within a few hours of baking.
Let your imagination run free with the flavourings – stirring chopped chives, coriander, parsley, garlic or caramelised onion into the mix – or serve them simply adorned with black nigella (or other) seeds.