Grilled Portabella Burgers with Garlic Mayo

Ingredients

• 6 oz Highline Mushrooms Portabella Grill Caps, stems and gills removed
• ⅓ cup mayonnaise
• 1 tsp chives, chopped
• ¼ tsp garlic powder
• ¼ tsp fine sea salt
• 1 small red onion, sliced (keep rings intact)
• 4 burger buns
• 4 lettuce leaves
• Olive oil, for grilling
• 2 tsp chili powder
• 1 tsp brown sugar
• 1 tsp fine sea salt
• 1 tsp garlic powder
• 1 tsp onion powder
• 1 tsp smoked paprika
• ½ tsp ground black pepper
• ⅓ cup extra virgin olive oil