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1
To cook the beets, adjust the oven rack to the middle position and preheat the oven to 400F.
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2
Place the beets in a baking dish large enough to fit them in a single layer.
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3
Drizzle the beets with the olive oil, season them with the salt and pepper, and toss to coat the beets with the seasonings.
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4
Cover the dish with foil and place it in the oven to cook the beets for about 1 hour (for medium-size beets), until they are easily pierced with a knife.
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5
Remove the beets from the oven, and remove the foil, being careful not to burn yourself from the steam that will rise.
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6
Set the beets aside until they are cool enough to touch.
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7
Rub the beets with a clean dishtowel to remove their skins and cut the beets into 1/2-inch-thick slices.
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8
Stack a few slices on top of each other and cut the slices into 1/2-inch batonettes, and then into 1/2-inch cubes.
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9
Transfer the cubed beets to a medium mixing bowl.
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10
(The beets can be prepared to this point up to one day in advance.
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11
Transfer to an airtight container and refrigerate until youre ready to serve them.
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12
Bring the beets to room temperature before serving.)
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13
To make the dressing, combine the vinegar, mustard, salt, and pepper, and whisk.
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14
Add the oil in a slow steady stream, whisking constantly to form an emulsion.
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15
Pour the dressing over the beets, and toss to coat them with the dressing.
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16
Add 2 tablespoons of the horseradish and toss again to distribute the horseradish.
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17
Taste for seasoning and add more salt, pepper, or horseradish, if desired.
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18
Transfer the beets to a serving platter and scatter the mache leaves over the top, or use scissors to snip the chive in 1-inch pieces over the beets.
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19
Serve at room temperature.
