Heat the oven to 160°C/325°F/gas mark 3. Lay the filo onto a flat surface. In a bowl combine the oil with the melted butter and brush each filo sheet. You’ll need to work quickly at this stage to prevent the filo pastry from drying so layer the buttered pastry sheets onto a 22cm loose-bottomed tart tin, overlapping each sheet at a slightly different angle.
Drizzle a little oil on a frying pan and add the smoked bacon. Fry for 2 minutes then add the leek and onion and cook for a further 2 to 3 minutes, until the leek and onion have softened. Remove from the heat and transfer to a small bowl. Leave to cool for 5 minutes.
In a separate bowl combine the ricotta, lemon zest and seasoning, add the warm bacon and stir. Pour the beaten eggs over the mixture and add the parsley, then stir to combine. Pour the mixture into the tart case. Scatter with blue cheese.
Bake for 30 to 40 minutes, until just set. Cool slightly, slice and serve with a fresh salad leaves or sautéed spinach.