• Trim the root end from the romaine heart; coarsely chop the leaves.
• Peel and thinly slice enough onion to measure ¾ cup [1½ cups].
• Peel the cucumber, if desired, and trim the ends; cut the cucumber lengthwise into quarters, then crosswise into enough ½-inch pieces to measure 2 cups [4 cups].
• Cut away the core from the tomato; coarsely chop the tomato.

For the shrimp or jumbo shrimp:

• Rinse the shrimp, then drain on a paper-towel-lined plate and pat dry. Season lightly with salt and pepper.

In a large frying pan over medium-high heat, warm 1 tablespoon [2 TBL] oil until hot but not smoking. Working in batches if needed, add the shrimp and cook, turning once, until firm and cooked through, 1 to 2 minutes per side for regular shrimp, 2 to 3 minutes per side for jumbo shrimp. Transfer to a plate. Add more oil between batches if needed.
While the shrimp cook, prepare the dressing.

For the scallops:

• Pat the scallops dry with a paper towel; season lightly with salt.

In a large frying pan over high heat, warm 2 to 3 teaspoons oil until hot but not smoking. Working in batches if needed, add the scallops and cook without turning until golden brown on one side and starting to turn opaque, 2 to 4 minutes. Turn and continue cooking for about 30 seconds for translucent or 1 to 2 minutes for opaque. Transfer to a plate. Add more oil between batches if needed.
While the scallops cook, prepare the dressing.

In a small bowl, stir together the cocktail sauce base and 2 tablespoons [¼ cup] oil. Season to taste with salt and pepper.

• Cut the lemon into wedges for garnish.
• Strip the basil leaves from the stems; coarsely tear or chop the leaves.

In a large bowl, toss together the romaine, onion, cucumber, tomato, and basil. Season to taste with salt and pepper.

Transfer the salad to individual bowls, top with the shellfish, and drizzle with as much cocktail sauce dressing as you like. Serve the lemon wedges and any remaining dressing on the side.