Slice the top off the loaf of bread and remove the insides with your hands or a sharp paring knife, leaving about a ¾ inch thickness on the inside shell; set bread shell aside.
Melt the butter with the oil in a medium skillet over moderate heat.
Add in the onions, stirring to coat with the butter mixture.
Saute for about 3 minutes, until the onions begin to soften.
Sprinkle with salt, sugar, and thyme, and saute for another 10 minutes, until the onions begin to turn golden, being careful that they don’t burn.
Transfer the contents of the skillet to a mixing bowl and let cool.
In the bowl of an electric mixer, beat the cream cheese and mayonnaise together until smooth.
Fold in the onions and Gruyere cheese; stir until blended.
Spoon into the bread shell and serve immediately with chips or veggies or baguette slices.
OAMC: Cover and freeze for up to 2 months; when ready to serve, defrost the dip overnight in the refrigerator; preheat oven to 350°; bring the dip to room temperature for 20 minutes, bake for 20 minutes, until bubbling; serve immediately.