Use a mandolin on the thinnest setting if you have it, or just slice by hand.
In a small bowl or jar, whisk together all ingredients for the Orange Vinaigrette Dressing.
Peel the mango, cut it in quarters length-wise and cut out the hard center section. Then cut each quarter in thin strips or bite-size cubes.
Thinly slice the cucumbers.
Peel and very thinly slice the red onion.
When ready to serve, remove the seed from the avocados and gently peel. Slice into thin slices.
Put the lettuce, mangos and cucumber slices in a bowl. Add desired amount of dressing and toss.
Add avocado slices, season with salt and pepper and serve.