Preheat the oven to 220oC/425oF/gas 7. Place a 30cm non-stick ovenproof frying pan on a high heat, with a smaller non-stick pan on a medium heat alongside. Pour 1 tablespoon of olive oil into the larger pan. Peel and roughly chop the onions, adding them to the larger pan as you go. Roughly chop two- thirds of the thighs, finely chop the rest, and add to the onion pan. Cook for 6 minutes, or until golden, stirring occasionally. Meanwhile, place the mushrooms in the dry pan, tearing up any larger ones. Let them toast and get nutty for 4 minutes, then tip into the chicken pan and strip in half the thyme leaves.
Remove the pan from the heat, add a pinch of sea salt and black pepper, then stir in 1 tablespoon of red wine vinegar and 150ml of water. Working quickly, roll out the pastry so it’s 2cm bigger than the pan, then place it over the filling, using a wooden spoon to push it into the edges. Very lightly criss-cross the pastry, then brush with 1 teaspoon of olive oil. Poke the remaining thyme sprigs into the middle of the pie. Bake at the bottom of the oven for 15 minutes, or until golden and puffed up. Easy!