Toss the potato rounds, garlic bulb, Cajun spice, 1 T olive oil, and vinegar. Season to taste. Arrange in a flat layer in a roasting pan and roast at 350 for 1 hour.
Remove from heat, squeeze the garlic out of the bulb into the yogurt, mash together and season to taste.
Plate the dressing, top with the potatoes, dress with the remaining olive oil, garnish with the scallions, and serve hot.