Melt the butter in a large, heavy dutch oven or cast iron skillet over medium heat. Add the onions and garlic clove and sprinkle with a bit of salt. Stir the onions around to coat them with butter, then cook for about 5 minutes, stirring often, until the begin to soften. Reduce the heat to low and maintain a steady temperature for 2-3 hours. Stir the onions every 20-30 minutes and adjust the heat as necessary. Once the onions are a deep caramel color, transfer them to a bowl and remove the garlic clove.
In a medium bowl, combine the yogurt, mayonnaise, vinegar, celery salt, onion powder and black pepper. Stir until smooth. When the onions have cooled to about room temperature, stir them into the yogurt mixture. Taste and adjust seasoning as necessary.
Transfer to a serving bowl, cover with plastic wrap, and refrigerate until ready to use. Serve along side salty potato chips, fresh veggies, or endive leaves.