Sliver red onion and soak in a bowl of cold water to reduce its bite if you like. Meanwhile, slice peel off orange, following curve of fruit; then slice thinly into wheels. Slice each wheel in half.
In a small bowl, blend lemon juice with olive oil, basil, and salt and pepper to taste.
Peel and slice avocado thinly crosswise. Lift onion from water and pat dry.
Spoon 1 or 2 slices of avocado, 1 slice orange, and a few slivers of onion onto each lettuce leaf and drizzle with vinaigrette. Top with flake salt.