In a large skillet over medium heat, melt the butter. Once hot and bubbly, add the onions. Season with dried thyme and salt. Cook the onions for 5 minutes, until softened.
Reduce the heat to medium-low and cook the onions until dark brown and caramelized, 30-40 minutes, stirring often. If the pan becomes dry, add a splash of water as often as needed.
Stir in the tamari. Cook for 2-3 minutes, until the onions are dark and jammy. Remove from the heat and let the onions cool in the pan.
Make the dip
In a medium bowl, stir together the cooled caramelized onions, yogurt, mayonnaise, and garlic powder until well combined. Serve immediately for the creamiest consistency.
Notes
Storage: Store the dip in an airtight container in the refrigerator for up to 3 days. The dip will thicken up in the fridge.Make it ahead: This dip is best served immediately, as it thickens in the fridge over time. My recommendation for making this dip in advance is to caramelize the onions ahead of time (up to 5 days in advance). Then the day before, mix together the dip. The day of, stir once more and serve.