Preheat the oven to 200°C.
Salt aubergine slices in a shallow dish. Combine the passata, sugar and seasoning in a saucepan and bring to a simmer over a medium heat. Simmer for 4-5 minutes until thickened, then remove from the heat and set to one side. Remove the aubergine slices and dry off the excess water and salt.
Layer a quarter of the aubergine slices in a shallow round, medium sized baking dish and top with some of the sauce, some of the grated Gruyère, seasoning and pinches of the spices. Continue layering in this fashion, finishing with a thin layer of the passata on top of the final layer of aubergine slices.
Drizzle the olive oil on top and bake in the oven for 30-40 minutes until the cheese has melted and the aubergine is soft and cooked. Spoon onto serving plates and garnish with a sprig of sage leaves, serve immediately.