Lamb shank tagine with steamed couscous

Ingredients

• 2 tablespoons olive oil
• 4 lamb shanks
• 20 grams butter
• 2 onions, peeled, finely sliced
• 3 cloves garlic, roughly chopped
• 2 teaspoons ras el hanout
• 1 teaspoon ground ginger
• 1 teaspoon ground cumin
• 1 teaspoon ground cinnamon
• 1/2 teaspoon ground turmeric
• 2 1/2 cup water
• 1 cup raisins
• 1/4 cup honey
• 1 cup instant couscous
• 1 preserved lemon skin, finely sliced
• 1 tablespoon coriander
• natural yoghurt, to serve