Overlap alternate layers of the onion, tomato and avocado.
Garnish with basil sprig.
To prepare the basil vinaigrette, place remaining ingredients except olive oil in a blender; cover and blend for 5 seconds.
Remove center cap from blender cover. With the blender running at slowest speed, slowly pour olive oil through the opening of the cover. Run blender until oil is fully incorporated.
Taste and adjust seasoning. Transfer vinaigrette to an appropriate storage container and refrigerate to store.
Drizzle basil vinaigrette over the salad. Accompany salad with a warm loaf of crusty sourdough bread and European-style butter.