In small bowl, mix sour cream, mayonnaise and mustard until blended.
Spread half of sour cream mixture on cut side of bottom half of bread; spread remaining mixture on cut side of top half of bread. On bottom half of bread, layer roasted peppers, roast beef and lettuce. Cover with remaining half of bread. Wrap tightly with plastic wrap; refrigerate 1 to 2 hours. When ready to serve, cut sandwich into 8 pieces.