In a small bowl, whisk together olive oil, lime juice, garlic, and cayenne. Season with salt. Scoop out the flesh from avocado halved, reserving shells and chop. Transfew to a bowl and add bell pepper, tomatoes, scallion and chopped cilantro. Drizzle with dressing and season with salt. Gently stir to combine. Spoon mixture into reserved shells. Garnish with whole cilantro leaves and serve immediately.