Chicken Breast Sandwich, Onions, Watercress, Paprika Aioli

I have been looking for so many different ways to serve chicken and then I spotted this! Served these for lunch one day and they were awesome! Due to the computer - had to cut things short - onions caramelized and the Spanish paprika is sometimes labeled Pimenton Picante or Pimenton de la Vera Picante, it can be found in specialty food store or from tienda.com. Also, you may want to make the sandwiches on sourdough bread (1/2" thick)

• Ready In: 1 hr
• Serves: 6
• Yields: 6 sandwiches

Ingredients

• 1⁄2 cup mayonnaise
• 1  tablespoon fresh lime juice
• 1  teaspoon  hot spanish  smoked paprika
• 1  garlic clove, pressed
• 2  tablespoons extra virgin olive oil, plus more
•  extra virgin olive oil, for brushing on roll
• 2  tablespoons  vegetable oil
• 2  lbs onions, halved lengthwise, thinly sliced crosswise (about 7 cups)
• 3  boneless skinless chicken breast halves, cut crosswise in half (8 oz each)
• 6  sourdough rolls
• 3  cups  lightly packed watercress or 3  cups arugula

Directions

1. Whisk mayonnaise, lime juice, Spanish paprika*, & garlic in small bowl.
2. Gradually whisk in 2 tablespoons olive oil.
3. Season with salt and pepper.
4. Aioli can be made 2 days ahead; cover prepared aioli and chill.
5. Heat vegetable oil in heavy large skillet over medium-high heat.
6. Add onions, sprinkle with salt, and cook until beginning to brown, stirring often, about 10 minutes.
7. Reduce heat to medium and continue to cook until onions are soft and deep golden brown, stirring often, about 20 minutes longer.
8. Season with salt and pepper.
9. Onions can be made 2 hours ahead; let stand at room temperature.
10. Prepare barbecue (medium-high heat).
11. Place chicken breasts between 2 sheets of plastic wrap.
12. Pound with mallet to 1/3-inch thickness.
13. Sprinkle with salt and pepper.
14. Grill chicken until cooked through, about 3 minutes per side.
15. Transfer to work surface.
16. Brush cut side of rolls with extra-virgin olive oil.
17. Grill rolls, cut side down, until grill marks appear, about 1 minute.
18. Spread rolls with some of aioli.
19. Assemble sandwiches, dividing chicken, caramelized onions, and watercress among rolls.
20. Serve warm.