Corned Beef Buns with Horseradish, Parmesan and Walnut Sauce

Ingredients

• 4 white rosetta or other white bread rolls
• 400 gm cooked corned beef (or brined ox tongue), thinly sliced
• Freshly grated horseradish, to serve
• Bitter leaves and gherkins (optional), to serve
• Walnut sauce
• ½ garlic clove, thinly sliced
• 1 tbsp olive oil
• 20 gm crustless day-old sourdough
• 1½ tbsp milk
• 100 gm walnuts
• 35 gm parmesan, finely grated