In a covered saucepan, gently poach scallops with garlic, vegetable oil and water for 3 to 4 minutes, turning once. Using a slotted spoon, remove scallops to a ceramic or glass bowl. Rapidly boil down liquid in a pan to about 1/4 cup. Cool and combine this liquid with remaining ingredients, except avocada and lettuce. Pour over scallops in bowl. Refrigerate for 1 to 2 hours. Just before serving, peel and dice avocado. Toss gently with the scallops. Serve on the lettuce garnished with additional mint sprigs.
