Cut bell pepper in half lengthwise; discard seeds and membranes. Place pepper halves, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil 15 minutes or until blackened. Place in a zip-top plastic bag; seal. Let stand 15 minutes. Peel and cut each piece in half.
Cut loaf in half lengthwise. Spread Pesto Mayonnaise evenly over bottom half of loaf; sprinkle with feta. Arrange bell pepper, tomato, and onion over bottom half of loaf; top with remaining half. Cut loaf into 4 sandwiches.
Totals include Pesto Mayonnaise.