1. Preheat an oven to 400 degrees F.
2. Peel skin off of about 3-4 beets. Poke a few holes in each one and lightly coat with olive oil. Roast for around 45-60 minutes until the beets are soft. Let cool 5-10 mins and then grate on a cheese grater.
3. In a saucepan, over medium heat, fry onions in oil until softened and starting to caramelize – stir often.
4. Add the remaining ingredients of the relish and let it cook down until most of the liquid evaporates. Make the cornstarch slurry by combining corn starch and water, and pour into the relish. Let it cook in for a few minutes. Remove the relish from heat and let cool. Store in the fridge!
5. Combine the burger ingredients and form 6 equal sized patties. Grill on medium heat until 155-160 degrees F.
6. Assemble burgers with the lightly toasted bun, burger patty, cheese, and finish off with the relish.