Heat oil in a large nonstick skillet over medium heat. Add onion and next 6 ingredients (through garlic) to pan; cook 30 minutes or until onions are golden, stirring occasionally. Remove from heat; let cool.
Combine sour cream, mayonnaise, and horseradish; spread evenly over cut side of top half of bread. Arrange lettuce over bottom half of bread. Arrange onions and Simply Roasted Pork over lettuce. Top with top half of bread. Cut stuffed loaf into 4 pieces.