Prepare the beets: Remove greens, leaving about an inch of stem. Don't cut the taproot. Wash gently, being careful not to break the skin, and mix in a bowl with olive oil, kosher salt and pepper.
Wrap 2-3 beets in a small baking paper package. Repeat until the beet mixture is all used. Optionally, add one clove of garlic in each package for extra taste. Place onto the baking sheet and roast for 45-50 minutes, until tender.
Unwrap the packages and let the mixture completely cool. Peel the beets, cut in chunks and place in a bowl.
In a separate bowl, mix cider vinegar, mustard, thyme, olive oil, kosher salt and pepper. Add the beet cooking liquid in the baking package and mix until fully incorporated.
Combine dressing and beets in the bowl and toss. Refrigerate for 20 minutes before serving for optimal texture and taste.