Bell peppers are cooked down and accented with fresh basil and olives.
Heat oil in skillet over medium heat. Add whole garlic cloves; gently cook over low heat until golden, 8 to 10 minutes. Add onions; cook over medium-low heat, turning often, until they soften, about 5 minutes.
Add bell peppers, salt, pepper, and rosemary to skillet; stir to coat with oil, and cook, covered, over low heat, stirring occasionally, until peppers are soft and juices are released, 30 to 35 minutes. Stir in vinegar and olives. Cook 5 minutes. Just before serving, roughly tear basil leaves, reserving two for garnish. Stir in torn basil. Serve warm or at room temperature garnished with reserved basil.
The juice content of peppers varies. If peppers seem too juicy toward the end of their cooking time, remove the lid sooner and cook off more liquid.