1) For the marinade: put all the ingredients in a medium bowl and whisk until smooth. Reserve 60ml of the marinade in a small bowl.
2) For the scallops: add the scallops to the bowl with the marinade. Cover and refrigerate for up to 30 minutes. Remove the scallops from the marinade and pat dry with paper towels. Discard the marinade. Coat a large heavy saute pan with extra-virgin olive oil and put over medium-high heat. Add the scallops in a single layer. Cook until golden on one side, about two minutes, depending on size. Turn and brown on the other side, another two minutes. Drain on paper towels and set aside to cool slightly.
3) For the caramelised avocados: coat the bottom of a small, nonstick frying-pan over medium heat, with extra-virgin olive oil. Sprinkle one side of the avocado slices with sugar, and salt and pepper, to taste. Add them to the skillet and cook until the sugar is caramelized and golden, about two minutes. Turn gently and cook for another 30 seconds. Remove to a plate and set aside.
4) For the salad: put the baby lettuces in a bowl with half of the grapefruit segments. Add the reserved 60ml of marinade and toss until coated.
5) To serve: put the dressed lettuces on a serving platter. Top with the caramelized avocados, remaining grapefruit segments, and scallops. Serve additional vinaigrette on the side.