1Preheat the oven to 350°F.
2In a large ovenproof skillet set over medium heat, melt the butter with the oil. Sauté the onions for about 15 minutes, until they are soft and golden. Spread the onions over the bottom of the pan and take it off the heat, or spread them into an 8" or 9" round cake pan or pie plate that has been sprayed with nonstick spray.
3In a medium bowl, combine the flour, baking powder, and salt. In a small bowl, stir together the milk, oil, and egg. Add the milk mixture to the flour mixture and stir until just blended.
4Spread the batter over the onions and bake for 35-40 minutes, until pale golden around the edges, cracked on top, and springy to the touch.
5Cut into wedges or squares and serve warm.