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1
Preheat the broiler of your oven.
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2
In a 10-inch heavy skillet warm 2 tablespoons of the olive oil over medium heat. Add the onion and cook, stirring frequently, until soft and translucent but not browned.
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3
Add the kale, 1 tablespoon of the oil, and a pinch of salt, and stir until just wilted. Reduce the heat to low, cover the pan, and cook for 15 minutes, stirring occasionally.
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4
The kale should be very tender, and the onions should have taken on a rosy brown color. Set aside.
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5
Break the eggs into a medium bowl, and beat them well with a fork. Add the cheese and salt, and beat again to blend. Add the kale and onion mixture, and beat to mix well.
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6
Wipe any burnt bits from the skillet, and add the remaining 1/2 tablespoon oil. Place the pan over low heat.
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7
When it is warm, pour in the egg mixture. If necessary, use the fork to gently push the kale around a bit, so it is evenly distributed. Cook over low heat until the bottom of the frittata is lightly browned - tease the edge up with a heatproof spatula and peek underneath - and the top looks mostly set.
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8
Remove the pan from the heat and slide it under the broiler until the top is nicely browned, 30-60 seconds. Don't walk away from it while it's under the broiler; it cooks very quickly.
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9
Cut into wedges, and serve warm or at room temperature, with additional salt, if needed.
