Finely dice onion. Tear lettuce into bite–sized pieces. Remove hard stems from rocket.
With a sharp knife, remove skin and white pith from grapefruit. Separate grapefruit into segments.
Drizzle scallops with lemon juice and sprinkle with salt and pepper. Heat butter in a nonstick pan and cook scallops about 2 minutes on each side; add vermouth and cook another 1 to 2 minutes. Remove from heat.
To make vinaigrette, whisk oil, vinegar, mustard and honey, until thickened slightly; add salt and pepper to taste.
Arrange lettuce leaves on individual plates and drizzle with vinaigrette. Top with diced onion and grapefruit segments. Arrange scallops on top.
The muscle from the large white scallop shells is particularly firm, but still tender. It has a mild, slightly sweet taste. A scallop shell also cntains a roe sack, the orange-red coral, that is prized by gourmets. Scallops are a good source of zinc and provide iron.