Rinse and dry the mache - set aside.
Using a serrated knife, slice off each end of the grapefruit so that just a little pink is showing. Set the grapefruit on a flat side and begin slicing off the skin vertically - you should be going around the grapefruit and removing the skin and membrane to show nice juicy grapefruit segments.
Next, segment each grapefruit slice by taking a serrated knife and cutting into the grapefruit just between the fruit and the membrane, separating the segments. Do this until all the fruit is freed. Squeeze the juice out of the remaining membrane into a small bowl.
Slice the avocado in half and remove pit. Then slice into quarters. Gently peel off the skin so that you have 4 quarters. Slice each of these lengthwise into 1/8 inch thick slices. Sprinkle with reserved grapefruit juice to keep from browning.
Heat a non-stick skillet or grill pan to high. Lightly oil it and place scallops flat side down on the pan. Watch them carefully - when they start to turn opaque about half way up their sides, flip them all over (this should take about 2 - 3 minutes) until golden brown then remove to a plate.
Toss the mache in half of the vinaigrette and lay out on a big platter or divide in half and serve on individual plates. Arrange grapefruit and avocado slices over the mache and top with the warm scallops. Drizzle remaining vinaigrette over all, making sure to get a little over the avocado and scallops. Serve immediately.
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