Chicken tagine with preserved lemons

Ingredients

• pinch of saffron threads
• 1 chicken stock cube
• 250ml just-boiled water
• 8 boneless, skinless chicken thighs
• 2 preserved lemons (size of a walnut in its shell), rinsed
• 1 tbsp olive oil
• 2 medium onions, sliced
• 3 garlic cloves, finely sliced
• 1 tsp ground ginger
• 2 tsp ground cumin
• 1 tsp paprika
• 1 tsp dried chilli flakes
• 2 tbsp cold water
• 600g medium potatoes, preferably Maris Pipers
• 3 large ripe tomatoes
• 50g pitted green olives in brine, drained
• 20g bunch of fresh coriander, leaves roughly chopped
• 20g bunch of fresh parsley, leaves roughly chopped
• flaked sea salt
• freshly ground black pepper