Mix garlic and olive oil in a small bowl and let sit for about 1 hour, or until needed.
Mix the yogurt and sour cream together in a bowl until combined. Add the onions and tomatoes and toss until well coated. Season with salt, freshly ground black pepper, and cayenne. Wrap in plastic wrap and chill in a refrigerator until needed.
Preheat a charcoal grill until coals are white and ashy.
Grill bread over hot charcoal, rotating as needed, until both sides are nicely toasted and slightly charred.
Brush grilled bread with the garlic olive oil and use a slotted spoon to transfer the tomato salad over top of each. Excess dressing can be spooned over if desired. Garnish with chives and serve immediately.