Bring a small saucepan of water to a boil. Add scallops and cook until firm, opaque and just cooked through, about 1 minute. Drain and rinse under cold water until cool.

Slice 1/4 inch off the bottom and top of the grapefruit; stand it on a cutting board. Using a sharp paring knife, remove the peel and pith. Hold the fruit over a medium bowl and cut between the membranes to release individual grapefruit sections into the bowl, collecting any juice as well. Discard membranes, pith, peel and any seeds. Transfer just the grapefruit sections to a serving bowl.

Whisk shallot, vinegar, mustard, salt and pepper into the bowl with the grapefruit juice. Whisk in oil in a slow, steady stream. Add the scallops and crab to the dressing; toss well to coat.

Add lettuce, tomatoes and avocado to the bowl with the grapefruit; toss to combine. Add the seafood and dressing; toss gently.

Ingredient Note: Be sure to request "dry" scallops (i.e., not treated with sodium tripolyphosphate, or STP) from your fish store. Sea scallops that have been subjected to a chemical bath are not only mushy and less flavorful, but will not brown properly.

Tip - Shopping for 2: What if you want just six cherry tomatoes, not a whole container full? Shop the salad bar at your local supermarket. The produce may cost a little more, but you're guaranteed not to waste any of it since you'll buy just what you need.