In a small bowl, whisk together olive oil, lime juice, garlic, and cayenne. Season with salt.
Scoop out flesh from avocado halves, reserving shells, and chop. Transfer 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.