In a large skillet, melt the butter over medium-low heat. Add the sliced onion and shallots. Sauté them gently, stirring occasionally, for approximately 30 minutes or until they become soft, golden brown, and caramelized. Set aside.
In a mixing bowl, combine the crème fraîche and mascarpone cheese. Mix them together until smooth and well incorporated.
Season the dip by adding Worcestershire sauce, garlic powder, sea salt, and black pepper to the mixture. Stir well to combine.
Add half of the caramelized onions and shallots to the dip and mix well.
Before serving, add the remainder of the caramelized onions and shallots on top of the dip and garnish with chives. Finish the dip with additional sea salt if desired.
Serve the dip with potato chips of your preference.