Mushroom & Cheese Katsu Burger

Ingredients

• 1 Red onion, finely sliced
• 1 tablespoon Sugar
• 1 tablespoon Vinegar
• 1 teaspoon salt
• 150g firm mozzarella cheese
• 4 large large Portobello mushrooms, stalks trimmed
• 4 tablespoons cream cheese
• half a cup flour
• 2 eggs, beaten
• 1 cup panko
• 4 tablespoons mayonnaise
• half teaspoon curry powder or 2 teaspoon sriracha sauce
• 2 cups shredded lettuce
• plain oil for shallow frying
• To serve, 4 toasted burger buns