Four cheese, leek and walnut gratin - delicious. magazine

Ingredients

• 1 tbsp olive oil
• 3 large leeks, washed and cut at an angle into 3cm slices
• 3 garlic cloves, finely sliced
• 60g walnut halves
• 50ml dry white wine (optional)
• 60g unsalted butter
• 60g plain flour
• 500g whole milk
• 1 tsp English mustard
• 30g parmesan or veggie equivalent, finely grated
• 50g mature cheddar, coarsely grated
• 125g ball mozzarella, torn
• 50g smoked cheese (see Know-how), coarsely grated
• Dash cider vinegar