• Preheat oven to 400°F/200 °C.
• Take 1 large piece of foil and wrap the beets tightly together, sealing the foil well. Roast for 1 hour or until tender. Remove the beets from the oven and leave to cool in the foil for 20 minutes this will give the steam time to dissipate and will prevent any steam burns.
• Cut the cooled beets into thin slices, and then into thin matchsticks, then chop into fine pieces. Add the vinegar, sugar, celery seed and salt to a medium saucepan. Over medium heat, bring the mixture to a simmer until the liquid is reduced by half. Mix the corn starch with 1 teaspoon water and whisk into the vinegar. Simmer until thickened and remove from the heat.
• To a bowl add the beets and pour over the warm vinegar, mix well.
• Allow to sit at room temperature before refrigerating. Transfer to clean jars.
• If not canning, the relish will keep for up 2 weeks sealed and refrigerated.

For canning/preservation:

• Per the USDA, jars do not need to be sterilized prior to using a water canning bath if being boiled for 10 minutes or longer, which applies in this case.
  
  This canning method is for a simple water bath. Make sure you are using proper canning jars (the ones in my pictures are just for the photos and not for canning/preserving). Place a round wire rack in the bottom of a deep pan (or canning pot). You must elevate the jars off the bottom of the pan to prevent them from breaking. Fill the pan with water (it should cover the jars by 1 to 2 inches). Bring the water to a boil, use tongs to carefully add the jars, cover with a lid and boil for 10 minutes. Remove the jars carefully.