Boil beets with skin on about 1 hour or until easily pierced with a fork. If a beet is older or really big, it may take longer. The smaller, younger beets may cook in 45 minutes. Let cool in cold water (or in the fridge overnight if you want to make the beets ahead). You can make them 2-3 days ahead and refrigerate until ready to use! Anyway, once they are at room temp or cool enough to handle, peel off skin. Most of the time, it should come off easily when you rub the beet with your hands (use kitchen gloves to keep from staining your hands).
Use a mandolin to julienne your beets or torture yourself and try to cut the matchsticks by hand. Just be prepared for really gnarly looking red finger nails if you don’t have a mandolin. Wear kitchen gloves if you are fond of your manicure. Place your sliced beets in a large bowl and top with 1/4 cup finely diced red onion.
Add in 1/4 tsp salt, 1/2 tsp sugar, 1 tbsp vinegar, and 3-4 tbsp real mayo to taste. Give it a good stir. You’re done!