Preheat oven to 350°F (175°C). Lightly grease a baking dish.
In a large bowl, mix cream cheese, sour cream, and mayonnaise until smooth.
Add ham, Swiss cheese, green onions, garlic powder, and Dijon mustard (if using). Stir to combine.
Transfer the mixture to the baking dish and spread evenly.
Bake for 20-25 minutes, until bubbly and golden on top.
Let cool for 5 minutes, then garnish with additional green onions if desired. Serve warm.