Place chicken between 2 sheets of heavy-duty plastic wrap, and pound to 3/4-inch thickness using a meat mallet or rolling pin.
Combine juice, mustard, oil, marjoram, thyme, 1 garlic clove, and chicken in a large zip-top plastic bag; seal. Marinate in refrigerator 2 hours, turning occasionally.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add remaining 4 garlic cloves, onion, sugar, fennel, crushed red pepper, and salt, and sauté 1 minute. Add roasted bell peppers; cook 5 minutes or until onions are tender, stirring frequently. Stir in vinegar and black pepper.
Prepare grill to medium-high heat.
Remove chicken from bag; discard marinade. Place chicken on grill rack coated with cooking spray; grill 5 minutes on each side or until done. Cool slightly; cut chicken into slices.
Spread cut sides of bread evenly with mayonnaise. Arrange cheese on bottom half of bread. Arrange chicken and pepper mixture over cheese. Top with top half of bread; press lightly.
Place stuffed loaf on grill rack; grill 3 minutes on each side or until cheese melts. Cut into quarters.