Cut the beets into pieces that are about 1-inch/2.5cm large, or bite-sized.
Place the beets into a glass storage container with a tight-fitting lid. Whisk together the oli, vinegar, mustard, and salt. Pour the dressing over the beets. Cover the storage container and gently shake it to disperse the dressing and coat the beets. Taste the beets and adjust the vinegar, mustard, salt and pepper to your liking.
Cover and transfer the beets to the fridge and allow them to marinate overnight (at least 8-12 hours). Marinated beets will keep for an additional 4 days in an airtight container in the fridge.