Using the side of a knife, mash the garlic with a pinch of salt to a smooth paste. Transfer the garlic to a bowl and whisk in the yogurt, mayonnaise, buttermilk and pepper. Season with salt. Divide the dip into two bowls.
In a small skillet, cook the oil, tomato paste and harissa over moderately low heat, stirring, until lightly browned, 2 minutes. Let cool slightly, then whisk the mixture into one bowl of dip. Serve the dips side by side.