Steak sandwich with caramelized onions & cheese

Ingredients

• 2 large onions
• 1 tablespoon olive oil (plus a drizzle for the steak)
• 1 tablespoon brown sugar
• 1 tablespoon balsamic vinegar
• 1/2 lb piece steak (sirloin steak)
• 1 pinch sea salt
• 1 pinch black pepper (to taste)
• 1 small loaf of bread
• 1 tablespoon grainy Dijon mustard
• 1 tablespoon mayonnaise
• 2 slices Frico Maasdam cheese
• 2 tablespoons pickled cucumber
• 2 handfuls arugula