Melt the butter with the oil in a large frying pan over medium-low heat. Add the onions and garlic. Season to taste with salt & pepper and cook for 30 minutes, stirring often, until golden. Take off heat and set aside.
Preheat the oven to 200°C Line a pie pan or loose bottomed tart tin with the pastry/pie crust. Line with greaseproof paper (wax paper) and fill with beans and bake for 15 minutes. Remove the paper and beans and cook for another 10 minutes until golden.
Lower the oven temperature to 180°C In a bowl, whisk together the eggs, yolks, mustard and cream. Stir in the onions and the chopped mint.
Spoon the mixture into the pie/pastry crust and bake for 20 minutes or until just set. Cool slightly before serving.