Blue Cheese Burger - La fournée dorée

Ingredients

• 4 Brioche Burger Buns
• ½ cup of red wine vinegar
• ½ cup of water
• 2 teaspoons of salt
• 2 teaspoons of sugar
• 1 medium red onion, peeled and thinly sliced
• 400g of ground beef
• 125g of blue cheese, cut into 4 slices
• 4 spoons of whole grain mustard (optional)
• Salad