Preheat oven to 425.
Heat butter in a large sauté pan over medium heat. Place onions, shallots, salt, and pepper in the pan. Slowly cook onions, stirring often until caramelized with a rich brown color.
Place all ingredients into a mixing bowl and stir until the mixture becomes smooth and creamy.
Spoon into a shallow soufflé dish (Recipe can be made two days ahead of time and refrigerated at this point).
Bake for 15 minutes.