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PREHEAT oven to 425°F. Cut a 1/2-inch-thick slice off top of bread loaf; set aside. Hollow out bottom of loaf with your hands, leaving 1/2-inch-thick shell. (Reserve removed bread for another use.) Brush mustard onto cut sides of bread.
FILL loaf with layers of half of the cheese, the ham, tomatoes and onions; sprinkle with remaining cheese. Cover with top of bread. Wrap in foil.
BAKE 30 to 35 min. or until filling is heated through and cheese is melted. Let stand 10 min. before cutting crosswise into 8 slices to serve.