Scallops are naturally sweet, and the radicchio provides the ideal contrast. Err on the side of caution with the lemon juice–remember you have the juices from the grapefruit segments, too.
Preheat oven to 400 F (200 C).
Melt butter in frying pan on medium heat. Add radicchio and cook for about 3 minutes, or until wilted. Add grapefruit and warm for 30 seconds on each side. Add lemon juice and parsley. Remove from stove.
Season scallops with salt and freshly ground white pepper. Heat oil in ovenproof frying pan on high heat. Add scallops and sear on one side for about 2 minutes or until dark golden brown. Turn scallops over, place pan in oven, and cook for 2 minutes or until doneu2013medium-rare (scallops still give a little when squeezed between finger and thumb).
Divide radicchio-grapefruit mixture among six warmed plates. Top each serving with a scallop.