Gourmet Steak Sandwich with Caramelised Onion - Ziggys

Ingredients

• 3/4 cup whole egg mayonnaise
• 1 large garlic clove, minced
• 3 large onions, peeled and finely sliced
• 40 g / 3 tbsp butter
• 2 tbsp brown sugar
• 1 tbsp balsamic vinegar
• 1/4 tsp salt & pepper
• 700g beef rump steak
• salt and pepper
• 1 tbsp vegetable oil
• 2 tomatoes, thickly sliced
• ½ avocado
• Jarlsberg cheese, sliced
• Iceberg lettuce, roughly chopped
• Mustard of your choice