Preheat the oven to gas 5, 190C, 170 fan and place a baking sheet on the top shelf to heat up.
Heat the oil and butter in a large non-stick frying pan until the butter froths. Add the onions to the pan and cook over a medium heat for 10 - 12 minutes, stirring occasionally, until softened and beginning to caramelize.
Unroll the pastry onto a sheet of greaseproof paper. Score a line 1.5cm in from the edge of the pastry to create a crust.
Spoon the onions onto the pastry and spread to fill inside the scored line and scatter over the camembert cubes. Brush the edges with milk.
Remove the hot baking sheet from the oven and place the tart on top. Return to the oven and cook for 15 - 20 minutes, until the edges are puffed and golden.
Cut into quarters and serve straight away.