Preheat the oven to 375 degrees.
Trim the tops and bottoms of the beet and wash off any dirt.
Place two overlapping sheets of foil into a baking dish with sides and then place the beets in the middle of the foil. Bring the foil up on the sides to make a bowl.
Make a separate foil pouch for the purple beets and place the purple beets in the middle of the foil and then bring the sides up to make another bowl.
Divide the garlic, bay leaf, thyme, salt, pepper, and olive oil between the two beet packets.
Create a packet by folding the foil together and sealing it. You don’t want the steam to escape.
Cook for between 1 and 2 hours, depending upon the size of the beets. Use a skewer to test for doneness - by poking right through the foil. When it slides through the beet easily, they are done. Remove from the oven and let them cool completely. (Do not open the foil when they come out of the oven or you can get a steam burn which is very painful.) When cool, refrigerate until the next day.