I love this recipe for many reasons. It combines some of my favorite things (sweet caramelized onions and gooey cheese), it never fails to delight guests, and it can be elegant or rustic. This is the kind of recipe that makes me feel like a much more put-together person than I am. I seems to me like the kind of thing you read about in a magazine food story, where the writer has shown up at some gorgeous farmhouse to write about the heirloom produce and the hosts casually bring this out with loafs of crusty bread just out of the wood-fired oven, like its no big deal. It takes a little more effort for me to look that non-chalant. That being said, this dish is a little work for a lot of reward. —The Runaway Spoon
Melt the butter and oil together in a sauté pan over medium heat. Add the onions and a generous pinch of salt and stir, cooking until they are soft and translucent. Add the white wine, stir, cover the pan and cook for a few minutes, until the wine has evaporated. Pour in the water, add the brown sugar and 2 teaspoons thyme, stir well and cover. Cook for 5 – 10 minutes over medium-low until the onions are deep brown and jammy, stirring a few times to prevent scorching. You can add a little water if needed. Spread the onions evenly on the bottom of a small baking dish, about 6 inches in diameter and 2 inches deep.
Pulse the gruyere in a food processor until it is small crumbs. Scrape the rind from the brie, cut into chunks and add to the food processor with the cream cheese, milk, cornstarch and remaining thyme. Process until you have a thick paste with a little chunky texture. Spread the cheese over the onions in the baking dish to cover evenly. You can now cover the dish and refrigerate for up to two days.
When ready to bake, preheat the oven to 350° and bake until golden and bubbly and warm though, about 20 minutes. Let the dip rest for 5 minutes before serving.