Heat a large skillet over medium-high heat until hot. Add the chopped bacon and cook for 3 minutes, or until crispy and lightly browned. Reserve 1 tablespoon of bacon for the topping.
In a medium bowl, using a hand mixer on medium-low speed, blend the cream cheese until smooth. Add the mayonnaise and sour cream and mix until combined. Stir in the mozzarella cheese, gruyere cheese, and bacon until combined.
Transfer the cheese mixture to the baking dish and cover with aluminum foil.
Bake for 35 minutes, until dip is bubbly in the middle and to browned around the edges.
Remove the baked dip from the oven and sprinkle with the reserved bacon pieces and green onions.