Using a serrated knife, cute bread crosswise into 1-inch slices, cutting to but not through the bottom crust.
In a small bowl, stir together butter, cheese, bacon, onions, mustard, and lemon juice. Spread mixture on both sides of each bread slice. Wrap loaf tightly in heavy foil.
For a charcoal grill, place loaf on the grill rack directly over medium coals. Grill, uncovered, about 10 minutes or until cheese melts and bread is heated through, turning once. (For a gas grill, preheat grill. Reduce heat to medium. Place loaf on the grill rack over heat. Cover and grill as above.)
