Parboil the potatoes in a large saucepan of boiling, salted water until just tender; roughly 10-12 minutes. Drain and allow to steam off to one side.
Meanwhile, melt the butter and olive oil together in a large, heavy-based saucepan set over a medium-low heat. Sweat the onions with a little salt for 10-12 minutes, stirring occasionally. Add the sugar and continue to cook over a reduced heat for 15-20 minutes until they start to caramelise. Remove from the heat at this point.
Preheat the oven to 170°C.
Whisk the eggs, milk and seasoning together in a large mixing bowl, fold in the onions and potato. Pour the mixture into a 7 inch nonstick cake tin, bake for 12-15 minutes until the egg is set. Remove and allow to cool in the tin a little before cutting portions. Serve from a wire rack, garnished with a sprig of basil leaves.
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