Put the kettle on. Fry the thighs in the oil in a casserole or wide pan with a lid to quickly brown. Stir in the whites of the spring onion with the flour and stock cubes until the flour disappears, then gradually stir in 750ml hot water from the kettle. Throw in the carrots and potatoes, bring to a simmer. Cover and cook for 20 mins.
Take off the lid and simmer for 15 mins more, then throw in the peas for another 5 mins. Season, stir in the mustard, green spring onion bits, herbs and some seasoning.