Preheat oven to 400 F or heat up your BBQ! Place mushrooms on a roasting pan, drizzle with a little olive oil, season with salt/pepper and broil for 20 minutes or barbeque directly on your grill for 5-10 minutes each site.
Heat olive oil in a large skillet. Add onions and sauté for 15-20 minutes on a medium heat until golden and caramelized. Add sugar, mix and heat through and remove from heat.
Mix mayo with sweet chili sauce.
Cut Portobellos in half. Spread spicy mayo onto bun, place mushroom topped with cheese, then caramelized onions, next add lettuce and top with the bun. Secure with a mini skewer and enjoy!