When ready to cook, set temperature to High and preheat, lid closed for 15 minutes. While the grill is heating, place a large cast-iron pan on the grill grate.
When the grill is up to temperature, melt the butter with the olive oil in the pan. Add the onions and 2 teaspoons of salt.
Stir, then close the grill and cook, stirring every 5 minutes, until the onions are very soft and dark brown, about 25-30 minutes.
Stir in the thyme and cook for 3 more minutes. Remove from the heat, add a fresh grinding of pepper, and allow the onions to cool.
In a mixing bowl, combine the sour cream, mayonnaise, and lemon juice. Stir in the grilled onions.
Taste, adding more salt and pepper as necessary. Serve with kettle chips or your favorite veggies. Enjoy!