In a skillet over medium heat, melt butter and add sliced onions. Sprinkle with salt and cook until golden brown (15-20 minutes).

In a bowl, combine cream cheese, sour cream, mayonnaise, garlic powder, salt, and pepper. Blend until smooth.

Fold in sautéed onions and shredded Gruyère until evenly mixed.

Preheat oven to 350°F (175°C). Transfer dip to an oven-safe dish, top with extra Gruyère if desired, and bake for 25-30 minutes until bubbly and golden.