Heat olive oil in a nonstick pan over medium-high heat, and cook and stir the onion and green chilli until tender, about 5 minutes. Stir in the garlic, lemon zest, and coriander; cook until fragrant, about 1 minute.
Push this mixture to the edges of the pan, and add the mango and pineapple to the center of the pan. Cook, without stirring, until the fruit begins to caramelize, about 5 minutes. Pour the lemon juice into the pan, and bring to a boil, stirring and scraping the browned bits of food off of the bottom of the pan with a wooden spoon. Remove from heat; stir in the butter, and stir to combine all ingredients evenly. Season to taste with salt and black pepper; serve warm.