To supreme grapefruit, cut off top and bottom to remove peel and membrane. Cut peel away from fruit, working knife down sides, leaving fruit exposed with all pith removed. Carefully cut each wedge of fruit away from internal membranes. Squeeze juice from membrane and use for recipe, tossing juice and wedges with other ingredients.
Trim off tops of radishes. Clean radishes well and pat dry. Heat butter and sesame oil with radishes until bubbling; turn heat down to a gentle simmer and cook until radishes are soft in center when pierced with a pick. Remove radishes and add grapefruit juice to butter/oil. Meanwhile, clean greens and pat dry. Plate greens, top with radishes and grapefruit and drizzle with warm butter/oil mixture. Season with pinch of salt and pepper and serve immediately.