Cheesy French Onion Chicken Sandwiches

 tablespoons extra virgin olive oil, plus additional for oiling grill or skillet

 large yellow onions

 bay leaf

 teaspoon kosher salt

 teaspoons Worcestershire sauce

 large boneless, skinless chicken breasts, pounded to a 3/4 inch thickness (or 4 thin-cut chicken breasts)

 ounces Gruyere or (Swiss) cheese

 whole wheat hamburger buns

 tablespoons Dijon mustard

 cup fresh spinach leaves