In a medium bowl, combine drained jar of chicken, mayonnaise, dijon mustard, cayenne pepper, diced white onion, lime juice and chopped cilantro. Add salt and pepper to taste.
In a toaster, toast the sourdough bread.
Place one slice of gruyere cheese, chicken salad mixture, and romaine on the toasted bread.
Serve immediately.
The chicken salad mixture can be stored in an airtight contain in the refrigerator for up to 3 days.