In a sauté pan on medium-low heat, melt butter and caramelize onions until golden brown, stirring frequently.
Add brown sugar, balsamic vinegar, garlic powder, and pepper. Simmer until brown sugar has dissolved.
Remove from heat and stir in yellow mustard and cayenne pepper. Hold refrigerated for service.
Set a panini press to medium heat. For each serving, spread 2 tablespoons of mustard on inside of 1 bread slice.
Place 4 ounces pastrami, 2 slices Havarti, 4 slices pickle, and 1/4 cup crispy fried onions on another bread slice. Close and butter outside with 2 tablespoons of butter. Place in panini press and grill until cheese is melted.
Remove from press and cut in half. Serve hot.
Don’t have a panini press? Use a griddle with a hot steak weight to create the same effect.
Try this sandwich with a house made coleslaw for an additional creamy texture.