In a large skillet on medium heat, heat 2 tablespoons olive oil. Add the onions,Member’s Mark™ Bay Leaf, and Member’s Mark™ Sea Salt. Cook until the onions turn a rich, golden brown color and a few turn lightly crisp, stirring often. Remove from heat, discard bay leaf, and stir in the Worcestershire sauce. Set aside.
Preheat a grill or large skillet over medium high heat and brush with olive oil to prevent sticking. Season chicken with Weber® Caramelized Onion Seasoning. Cook chicken until no longer pink and the interior reaches 160°F, turning once, about 4 minutes per side. Top each cooked chicken breast with a slice of cheese immediately after flipping.
Spread the bottom half of each bun with 1-1/2 teaspoons Dijon mustard. Top with fresh spinach, then the cheese-topped chicken, caramelized onions, and finally the top bun half. Serve immediately.