Scrub the beets well, and trim the tops and root ends.

If you are using fresh horseradish instead of prepared pickled horseradish, wash, peel, and finely grate the root. Mix with equal parts of vinegar by volume and a pinch of salt. (Skip this step if you have jarred prepared horseradish in the fridge.)

Peel, wash, and finely dice the onion. Measure out two cups of onion, and set aside any extra for another recipe.

In a large pot (big enough to hold all of the brine and cooked beets), combine the diced onion with the vinegar, sugar, horseradish, and seasonings (all of the ingredients except for the beets). Stir to combine and dissolve the sugar.

Wash your canning jars with hot, soapy water, and rinse well.

Fill your canning pot with enough water to cover the jars by at least 1″-2″.

Place the jars in the pot to keep them warm, and bring the pot up to a simmer while the beets are cooking.

Place the beets in a pot and add enough water to cover them.

Bring the pot to a simmer over medium heat.

Cover pot, and check occasionally to make sure all beets are covered by the water (stir them around a few times to make sure they cook evenly).

Cook for about 25-35 minutes, or until the beets are still somewhat firm, but can be pierced with a fork. Large beets will take longer to cook through, and small ones will be done more quickly, so adjust cooking time as needed.

Drain the beets and let them cool.

Bring the pot with the onion, vinegar, and horseradish mixture to a simmer over low heat while you peel and dice the beets.

If you have good knife skills and can dice them quickly, start the pot of brine before prepping the beets, but if you are slow, wait until you are mostly done with the beet prep so you don't over-reduce or boil away your brine!

When the beets are cool enough to handle, slip off the peels. The peels should rub or slip off easily if the beets are cooked enough, though you can use a peeler or paring knife for the tricky bits.

Cut the beets into 1/4"-1/2" slices (smaller for a fine relish, larger for a chunky one).

Cut those slices into baton/stick shapes, and then cut those again into small 1/4"-1/2" dice.

They don't have to be perfectly diced, but will make a prettier relish if they're relatively uniform.

Measure out 8 cups of the diced beets. If you have more, set them aside and enjoy them fresh in a salad so that you do not dilute the pickling brine.

Add the diced cooked beets to the hot brine mixture and gently stir until well-combined.

Return the pot to the stove, and bring up to a low simmer over medium-low heat.

Stir gently and simmer at a low boil for 12 minutes, stirring regularly so that it does not scorch.

Pack the hot beet relish into pint, half pint or 4 oz canning jars.

Leave 1/4" headspace at the top of the jars. Remove any air-bubbles with a butter-knife, chopstick, or debubbling tool.

Wipe the rims of the jars, and cover them with new canning lids.

Screw down the rings finger-tip tight.

Using jar tongs, place the jars into the hot water-bath in your canning pot.

Cover the pot and bring the water-bath up to a rolling boil.

Once the pot is boiling, start the processing timer.

Process the jars for 15 minutes (adjust for altitudes above 1000 ft). Turn off heat and let jars rest in the canner for 5 minutes before transferring to a wire rack or towel-lined countertop to cool.

Remove the jars of beet relish from the canner, and set on a folded towel or wire rack to cool. If the room is very cold or drafty, cover the hot jars with a kitchen towel so they cool evenly.

Leave the jars undisturbed overnight (or until completely cooled, at least 6-8 hours).

Check the lids for seals, remove bands, and wash the jars if they are sticky. Any jars that didn't seal can be reprocessed with new lids within 24 hours of processing, or put in the fridge or freezer to be used first.