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Mix the flours with the yeast and salt, add 1 tbsp oil, then pour in 320ml lukewarm water and mix well. You want a very soft dough – don’t worry if it looks a little wet, this will make a lighter focaccia. Knead for 5 mins if using a stand mixer, or 10 mins by hand, using a dough scraper if you have one and lightly oiling your hands and the surface. Transfer the dough to a lightly oiled bowl, cover with a damp tea towel and leave to rise until it has doubled in size.
Meanwhile, melt the butter in a large frying pan with the remaining oil, add the onions and a pinch of salt, and cook gently for 20 mins or until very soft. Pour in the vinegar and cook for a further 10 mins until sticky. Set aside to cool.
Oil an A4-sized roasting tin, scrape in the dough and reshape in the tin (see tip, below), gently pushing the dough into the corners. Scatter over the onions, cover with a piece of lightly oiled cling film and leave until puffed up.
Meanwhile, heat oven to 220C/200C fan/gas 7. Using your fingers, lightly dimple the dough all over, drizzle with a little oil and sprinkle with sea salt. Bake for 30 mins until golden brown. Can be made the day before and stored in an airtight container.