Christmas Eve savoy cabbage + roast garlic gratin

Ingredients

• 1 large savoy cabbage (approx 900g) or 2 smaller
• 3 onions, peeled and sliced into thin wedges (6ths or 8ths depending on size)
• 1 large garlic head
• 3 tbsp olive oil
• 180g cooked chestnuts, roughly chopped
• 300ml creme fraiche
• 1 teaspoon dijon mustard
• 1 tablespoon white wine vinegar
• 90g cheddar, grated
• 40g parmesan cheese, grated
• 100g breadcrumbs
• 15g sage leaves
• 1 tablespoon olive oil