Bake the beets until they are just tender. About 1 hour and 15 minutes. Cool the beets in the foil to room temperature. The beets can be prepared to this point up to 3 days in advance. Refrigerate in the foil packets.
Peel the beets and with a box grater, grate them coarsely. In a large, nonreactive pot over medium heat, warm the oil. Add the onion and garlic, cover and cook, stirring occasionally for 10 minutes.
Add the stock, the grated beets, vinegar, 1/4 cup of the sugar, the salt and pepper. Bring to a simmer, then partially cover and cook, stirring often until the beets are tender and the borscht is very thick 30 to 35 minutes.
Remove from the heat and puree in a Vitamix. Cover and chill until very cold. Spoon into individual serving containers and garnish with dill and cucumber.