Melt the butter in a 12-inch skillet over medium heat. Add the onions and toss them in the butter. Cut a piece of parchment paper the same size of the inside of the skillet. Place the parchment paper on top of the onions as they cook. Cook over low heat for about 25 to 30 minutes, stirring occasionally to pick up any brown bits on the bottom of the pan.
Pre-heat oven to 375ºF.
When the onions have softened and browned, add the sherry, Worcestershire sauce and thyme leaves, tossing well to combine. Season with salt and freshly ground black pepper and simmer for a few more minutes.
Remove the skillet from the heat. In a large bowl combine the softened cream cheese, sour cream, Parmesan cheese and ½ cup of the Gruyère cheese. Fold the caramelized onions into the mixture. Spread the mixture into a 2-quart casserole dish. Top with the remaining Gruyère cheese and bake in the 375ºF oven for 15 minutes.
Switch the oven setting to broil and leave the casserole under the broiler for 2 to 5 minutes, until the top has browned and is bubbly.
Garnish with the chopped chives and serve the hot dip with crostini.
Pre-heat the oven to 375ºF.
Slice the baguette into ¼-inch slices. Place the slices on a baking sheet, brush with olive oil and season with freshly ground black pepper. Sprinkle the grated Parmesan cheese on top.
Bake at 375°F for 10 to 12 minutes, until lightly brown.