Heat the oil in a large, thick-bottomed sauté pan over medium heat and add the onions.
Stir to coat all of the onions with oil. Cook, stirring occasionally, about 10 minutes until the onions start to soften. Add ¼ teaspoon salt and thyme and stir to combine. If desired, you can also add a small amount of sugar (about ¼ teaspoon) to help the onions caramelize but this is optional.
Continue to cook the onions, stirring occasionally, until they are golden brown and caramelized, another 35-45 minutes. As they cook, scrape up the fond (the brown bits) on the bottom of the pan and mix it into the onions. If the onions start to burn or catch bottom, add a small amount of water to the pan.
Once the onions are done, add a couple of tablespoons of water to the pan and stir to release all of the brown bits from the bottom of the pan.
Cool the onions, than transfer them to a cutting board and chop them.
Mix the yogurt, sour cream, garlic powder, Worcestershire sauce, ½ teaspoon salt and pepper together in a bowl. Set aside a small amount of caramelized onions for garnish and stir the remaining onions into the yogurt mixture until distributed throughout. Garnish with reserved onions and serve. For maximum flavor, make the dip a few hours before serving.