Cut the mangoes; discard the stones.
Score the mango flesh by making criss-cross cuts all the way to the skin. Scoop out the cubed flesh into a bowl.
Peel and roughly chop the onion.
Add the onion to the mangoes. Toss. Add the lettuce. Toss. Season with salt and pepper, drizzle with olive oil, toss and the salad is ready to serve.