Heat the oven to 190°C/170°C fan/gas 5. Heat a glug of oil in a large deep casserole with a lid. Add the vegetables and fry for 8-10 minutes over a medium-high heat to colour.
Add the cider, turn up the heat to high, then reduce for 2-3 minutes. Add the stock, chicken thighs and mustard. Put the lid on and cook in the oven for 40-50 minutes until the chicken is cooked through and tender. Add an extra glug of cider, stir in the crème fraîche and season.
Serve half the casserole, scattered with the freshly chopped parsley and a squeeze of lemon, if using.