Slow cooker chicken casserole - Good Food Middle East

Ingredients

• knob of butter
• ½ tbsp rapeseed or olive oil
• 1 large onion finely chopped
• 1 ½ tbsp flour
• 650g boneless, skinless chicken thigh fillets
• 3 garlic cloves, crushed
• 400g baby new potatoes, halved
• 2 sticks celery, diced
• 2 carrots, diced
• 250g mushrooms, quartered
• 15g dried porcini mushroom, soaked in 50ml boiling water
• 500ml stock made with 2 very low salt chicken stock cubes (we used Kallo)
• 2 tsp Dijon mustard, plus extra to serve
• 2 bay leaves