A while ago I put up a set of nested bowls made from a single piece of soft redwood (see pic below). These were made by holding the piece with a small (75mm) faceplate from each side and cutting through with a parting tool. The outer surface of the bowls was concave, which not everyone liked. So I have had another go with ripple sycamore with a convex outer surface, made the same way.
The bowls (except the smallest central one) have separate bases, this time of stained 3mm birch ply, glued in afterwards.
I attach pics & working notes.