Created by ellie on May 17, 2018
Step 1: Whisk first 4 ingredients (-garlic) in a small cup or bowl and then gradually whisk in 2 tablespoons olive oil. Season with salt and pepper. ***made be made 2 days ahead, covered and chilled.
Step 2: In a skillet over medium high heat heat the vegetable oil and hen add onions, sprinkle with salt, and cook until just beginning to brown, stir often (10 minutes). Reduce heat to medium and continue to cook until onions are soft and deep golden brown , stirring often (20 minutes). Season with salt and pepper. ***onions can be made 2 hours ahead - let stand at room temperature.
Step 3: Prepare barbecue (medium high heat). Place chicken breasts between 2 slices of plastic wrap. Pound with mallet to 1/2 inch thickness. Sprinkle with salt and pepper. Grill chicken until cooked through (3-5 minutes per side). Transfer to work surface. Brush cut sides of rolls with olive oil. Grill rolls, cut side down, until grill marks appear (1 minute). Spread rolls with some of aioli. Assemble sandwiches, dividing chicken, caramelized onions, and watercress (or arugula) among rolls. Serve warm.