Wrap each beet in aluminum foil, then place directly on a rack in the center of the oven. Roast for about 45 minutes, or just until they can be pierced with the tip of a paring knife. (Larger beets may require more time.) Use tongs to transfer the wrapped beets to a work surface; partially open them with the tongs to let steam escape. Let the beets rest until they are cool enough to handle.
Combine the salt and vinegar in a bowl large enough to hold the beets.
While the beets are still warm, discard the foil and peel their skins, which should rub off easily. Or use a paring knife, as needed. Halve or quarter the beets if they are on the larger side.
Add the beets to the bowl with the seasoned vinegar and toss thoroughly to coat. Marinate for at least 10 minutes.
When ready to serve, divide the beets among individual plates, then follow with the mustard greens, or toss them together and then divide among plates. Drizzle each portion with a little of the oil, and season with salt and/or pepper, as needed.
From food writer Emily Horton.
Tested by Luis Warner; email questions to firstname.lastname@example.org.
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Servings Per Container: 4; Calories: 140; Total Fat: 7 g; Saturated Fat: 1 g; Sodium: 12 mg; Carbohydrates: 6 g; Dietary Fiber: 23 g; Sugars: 12 g; Protein: 4 g.