Spring Onion Pie

"What my children eat on any given day is often a mystery, but they will always love something with eggs and cheese."
Preheat the oven to 400 degrees. Preheat a 10-inch cast-iron skillet in the oven. Separate the roots from the onion; discard the roots. Chop half of the onion.Whisk together the eggs and milk. Sift together the flour and the next 3 ingredients. Gradually add the flour mixture to the egg mixture, beating quickly for 20-30 seconds or just until smooth. (There should be no lumps.) Add the chopped onion. Let stand for 5 minutes.Carefully remove the hot pan from the oven. Add the butter and let stand until the butter melts. Put the pan on medium-high heat and pour the batter into it. Place the cheese and remaining whole onion on top of the batter and cook for 30 seconds to 1 minute, or until the edges start to set.Transfer the pan to the top rack of the oven and bake at 400 ° C for 22-25 minutes or until golden brown and fluffy. (The outer edges should be crisp, and the inner texture should resemble a custard filling. The pie will quickly deflate.) Serve immediately.