Easy one-pot chicken casserole

INGREDIENTS

• 8 bone-in chicken thighs , skin pulled off and discarded
• 1 tbsp oil
• 5 spring onions , sliced
• 2 tbsp plain flour
• 2 chicken stock cubes
• 2 large carrots , cut into batons (no need to peel)
• 400g new potatoes , halved if large
• 200g frozen peas
• 1 tbsp grainy mustard
• small handful fresh soft herbs, like parsley, chives, dill or tarragon , chopped