In a large bowl, combine the flour, yeast, sugar and 1 tsp salt. Make a well in the centre and gradually pour in 350ml tepid water, stirring and mixing the water into the flour. If the mix is a little wet, stir in a bit of extra flour; if a little dry, add a bit more water. The dough should be soft but not too sticky. Turn the dough out onto a floured work surface and knead for about 10 mins until the dough is springy but smooth. Put in a large floured bowl, cover with cling film and leave in a warm place to rise until doubled in size (this will take 45 mins-1 hr).
While the dough rises, heat the 2 tbsp oil and butter in a non-stick frying pan. Stir in the onions and cook over a medium heat for 10-15 mins until golden and softened. Leave to cool, then stir in the cheese and thyme leaves, and season.
Once the dough has risen and doubled in size, knock it back and divide into 12 balls. Flour a work surface and roll out the balls to about 10cm across. Spoon 1 heaped tbsp of the onion mixture in the middle of the dough and gather up the edges over the filling, pressing together to seal. Do this with all the balls, reserving a little onion mixture. Oil a baking tray and arrange the balls, seam-side down, in rows. Cover with oiled cling film and leave in a warm place to prove for another 30 mins.
Heat oven to 220C/200C fan/gas 7. Once the buns have risen a little more and are beginning to join together, remove the cling film, drizzle with the remaining oil and spoon over the leftover onion mixture. Bake for 10 mins, then turn the oven down to 200C/180C fan/gas 6 and cook for another 10 mins. The buns should be golden and crisp on the bottom. Cool before serving.