1. Chop one of the rosemary sprigs with garlic and add to a small glass jar. Add olive oil and stir - or shake if the glass has a lid on. Tie a string around the remaining 4 sprigs of rosemary so they can be used as a brush. Prepare the mushrooms by removing any dirt and the stems.
2. Add vinegar, water and sugar to a pan, bring to a boil and turn off the heat. Add the Pioppini or button mushrooms and leave them in the warm vinegar for at least 30 minutes.
3. Put the red peppers on a burning hot grill and grill on each side until the skin blisters and the peppers soften. Place the peppers in a bowl and cover with cling film. After 10 minutes peel the skin off and remove the stem and seeds. Cut them into 8 evenly sized strips.
4. Grill the Portobello mushrooms for 3-4 minutes with the gills facing down. Brush the top of the mushrooms with the rosemary and garlic oil, turn the mushrooms and brush the gills with oil. Add a slice of Castello Burger Blue on each Portobello mushroom and grill for 6-8 minutes until the mushrooms are soft. Toast the burger buns on the grill, remove from the grill and cut them in half.
5. Spread a nice layer of mayonnaise on each side of the buns. Add corn salad, a grilled Portobello mushroom with Castello Burger Blue, red pepper strips and pickled mushrooms. Finish with the top bun.