Pressure cooker chicken stew

Name: Pressure cooker chicken stew
Ingredients: 25g butter; 6 skinless chicken thighs cut into large chunks; 1 onion finely chopped; 2 garlic cloves crushed; 2 bay leaves; 2 carrots halved lengthways and cut into chunks; 1 large leek well washed and thickly sliced; 1 large potato chopped into bite-sized chunks; 2 tbsp plain flour; 100ml white wine or cider (optional); 600ml chicken stock; 1 tbsp wholegrain mustard; small handful of fresh soft herbs like parsley, tarragon or chives, chopped