Sweet Potato, Celeriac, Parsnip & Potato Gratin

Ingredients

• 60 g melted butter
• 3/4 cup sour cream
• 1/2 cup chicken consommé
• 1 medium brown onion peeled & finely sliced
• 1 clove garlic peeled crushed
• 2 tsp lemon thyme leaves
• 1/4 tsp nutmeg
• Sea salt and freshly ground black pepper
• 150 g sweet potato, peel & thinly sliced
• 150 g celeriac, peeled & thinly sliced
• 150 parsnip, peeled & thinly sliced
• 150 potato, peeled & thinly sliced
• 1 cup fresh sourdough breadcrumbs
• 2 tsp chopped parsley
• 1/3 cup (25g) finely grated Parmesan