Slow cooker chicken, ham and leek pie filling

INGREDIENTS

• 1 tsp vegetable oil
• 6 skinless, boneless chicken thighs
• salt and freshly ground black pepper
• 3 medium leeks
• 1 onion
• a few sprigs fresh thyme
• 1 tbsp butter
• 2 tbsp plain flour
• 350ml/12fl oz good-quality chicken stock
• 100g/31/2oz good-quality ham
• 100g/31/2oz crème fraîche