In a large bowl, beat the cream cheese, mayonnaise and mustard until smooth. Stir in the shrimp, crab, 1/3 cup Monterey Jack cheese and onions.
Cut the top fourth off the loaf of bread; carefully hollow out bottom, leaving a 1/2-in. shell. Cube removed bread; set aside.
Spoon seafood mixture into bread shell. Sprinkle with remaining cheese. Wrap tightly in heavy-duty foil and place on a baking sheet.
Bake at 350° for 25 minutes. Unwrap; bake 20-25 minutes longer or until cheese is melted and dip is heated through. Serve with vegetables and reserved bread cubes.