In a large non-stick pan, add the olive oil, sliced white onions, thyme and salt and pepper to taste. Sautee on low for 45 minutes-1 hour, stirring often. If onions begin to get too dark or dry, add a few tablespoons of water to the pan. When the onions turn a deep golden color, remove from heat and let cool slightly.
Add the sour cream, mayonnaise and cream cheese to a large bowl, along with both cheeses, reserving about a tablespoon of each cheese for garnish. Mix thoroughly.
Add the cooled onions into the sour cream mixture, reserving a tablespoon for garnish. Mix thoroughly.
Transfer the mixture to a medium baking dish, sprinkle the reserved cheeses on top and add the reserved caramelized onions to the center of the dip.
Bake at 400 degrees for 20-25 minutes until golden and bubbling.
Remove from the oven and garnish with torn basil.
