Put all the flatbread ingredients in a bowl with ½ tsp salt and 100ml warm water. Using your hands, bring together to form a dough, adding more water if needed. Tip onto a work surface and knead for 10 mins until smooth and elastic. Return the dough to the bowl, cover with cling film and leave to prove somewhere warm for about 1 hr, or until doubled in size.
Meanwhile, heat oven to 200C/ 180C fan/gas 6. Put the onion in a roasting tin, scatter over the nigella seeds, oil, thyme and some salt, and toss together. Roast for 25 mins, turning the onion every 5 mins or so to ensure even colouring and cooking. Remove from the oven and leave to cool. Can be made up to two days ahead.
Tip the dough onto a lightly floured surface. Divide it in two, then roll each piece out into a long oval shape. Transfer each flatbread to a baking sheet. Bake in the oven for 10 mins until puffed up and golden, then spread with onion marmalade and scatter with the roasted onions. Top with the ham, scatter over the crumbled feta, then return to the oven for 5-10 mins more. Scatter with parsley and spring onion, drizzle with a bit more oil, and serve.