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1
Whisk all of the ingredients together in a medium bowl until smooth.
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2
Cover and refrigerate until ready to serve.
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3
Heat the oven to 400 degrees F and arrange a rack in the middle.
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4
Heat the oil in a large oven-safe nonstick frying pan over medium heat until shimmering.
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5
Add the onion, season with salt and pepper, and cook, stirring occasionally, until softened and lightly browned, about 5 minutes.
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6
Add the asparagus, season with salt and pepper, and cook, stirring occasionally, until the asparagus is crisp-tender, about 5 to 6 minutes more.
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7
Meanwhile whisk the eggs, milk, cheese, horseradish, parsley, measured salt and pepper, and nutmeg in a large bowl until evenly combined.
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8
Turn off the heat, pour the egg mixture into the pan with the asparagus mixture, stir with a rubber spatula to combine, and spread the asparagus into an even layer.
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9
Transfer the pan to the oven and bake until the frittata is puffed and just set in the center, about 10 to 12 minutes.
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10
Remove the pan to a wire rack and let it cool slightly, about 5 minutes.
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11
Loosen the edges of the frittata with a rubber spatula and slide it onto a cutting board.
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12
Slice it into wedges and serve immediately, passing the horseradish sour cream and chives on the side.
