Blue Cheese & Caramelised Onion Portabello Mushroom Burgers

Ingredients

• 6 large portabello mushrooms (whole)
• 100 g butter (for onions)
• 150 g butter (for mushrooms)
• ¼ cup cooking pil
• 700 g onion, thinly sliced
• 1 Tbsp fresh rosemary, finely chopped
• 2 Tbsp balsamic vinegar
• ¼ cup brown sugar
• Sea salt & fresh black pepper
• ½ Tbsp garlic, finely chopped
• 200 g blue cheese, crumbled
• 6 hamburger buns
• Rocket (one bag)