Portobello Lamb Burgers

Ingredients

• 680 g / 1 1/2 lb ground lamb
• 1 Spanish onion, chopped finely
• 1 egg
• 125 ml / 1/2 cup flat parsley, chopped
• 10 ml / 2 tsp fresh oregano, chopped
• To taste salt and freshly ground pepper
• 4 portobello mushrooms, stems removed
• 4 hamburger buns, toasted
• 50 ml / 1/4 cup mayonnaise
• 2 cloves garlic, minced