Original recipe yields 24 servings
Preheat oven to 350 degrees F (175 degrees C).
Heat vegetable oil in a skillet over medium heat. Cook and stir onion until tender, 5 to 7 minutes.
Transfer onion to a large mixing bowl.
Stir corned beef, cream cheese, mayonnaise, sour cream, Swiss cheese, and garlic powder into onion. Mix until thoroughly combined.
Slice a piece of bread from the top of the Italian bread and hollow the center of the loaf, creating a long bread bowl. Leave at least 1/2 inch of bread on the sides. Reserve bread chunks from center of loaf for dipping if desired.
Spoon the corned beef mixture into the bread bowl.
Wrap the filled bread bowl in aluminum foil and place on a baking sheet.
Bake in preheated oven until the dip is fully cooked and bubbling, 1 to 1 1/2 hours.