1Place the beets and kombu in a 1-quart jar, leaving at least 1/2 inch of room at the top of the jar.
2Make the brine: Place the vinegar, sugar, water, and salt in a small saucepan, whisk to dissolve the sugar and salt, and bring to a rapid simmer.
3Immediately pour the brine over the beets, making sure to cover them completely but leaving 1/4 inch of room at the top of the jar. Let cool to room temperature, about 1 hour.
4Cover the jar with a tightfitting lid. Shake the jar or turn it upside down to evenly distribute the brine, then place it in the refrigerator for at least 1 day and preferably 1 week before using. (The pickled beets can be kept in the refrigerator for up to 2 months.)