Step 1Leaving 1 inch of stem and all of the roots, steam beets until they're tender-firm when pierced with a knife, 20 to 25 minutes. Let cool, then slip off skins. Cut beets in quarters or sixths, keeping different colors, if using, separate.
Step 2Pound garlic and salt in a mortar until smooth, then stir in lemon juice and olive oil. Dressing should be a little on the tart side. Toss each kind of beet in enough dressing to coat lightly.
Step 3Arrange beets on a platter and garnish with arugula leaves. Just before serving, tuck cheese among greens, along with olives. If any dressing remains, spoon it over cheese. Season with pepper.