Preheat oven to 375 degrees F. Spread 1 cup of the kosher salt in an ovenproof skillet or baking dish. Put the beets on the salt and cover with the remaining salt. Bake until the beets are tender, about 90 minutes.
When cool enough to handle, remove the beets from the salt and peel (discard salt). Set aside to cool completely. Grate them into a mixing bowl using the coarse holes of a box grater. Add the tehina, lemon juice, olive oil, dill, mint, and 1/2 teaspoon salt. Mix well to blend. Serve at room temperature or chilled.
Basic Tehina Sauce
Break garlic head into cloves and place unpeeled cloves in a blender. Add lemon juice and 1/2 teaspoon salt. Cover; blend for a few seconds until a coarse puree. Let stand for 10 minutes to allow the garlic to mellow.
Pour the mixture through a fine-mesh sieve set over a large mixing bowl, pressing on the solids to extract as much liquid as possible. Discard the solids. Add the tahini to the strained lemon juice in the bowl, along with 1 teaspoon salt and the cumin. Whisk the mixture together until smooth. Add 1 1/2 cups ice water, 2 tablespoons at a time, whisking vigorously until mixture is smooth and velvety (mixture may separate at first, but will become smooth as you whisk). Adjust seasonings. Cover and chill up to 1 week. Makes about 3 1/2 cups.