1. Spread out the flour in a pie plate lined with wax paper or parchment. Season the scallops with salt and pepper to taste. Working in batches, toss the scallops in the flour, lifting the wax paper on both sides to move the scallops around; transfer the scallops to a strainer and shake off the excess flour.

2. Heat 2 tablespoons vegetable oil in a large skillet over medium heat until hot. Add the scallops to the skillet and saute for 2 to 3 minutes per side, or until just cooked through. Transfer the scallops to a plate using tongs and set aside to cool to room temperature.

3. Put the grapefruit juice in a small saucepan or skillet and simmer until it has reduced to 2 tablespoons; transfer it to a small bowl. Finely chop the shallot (about 2 tablespoons); add it to the reduced grapefruit juice along with the rice vinegar, ¼ teaspoon salt, and sugar. Whisk until the sugar and salt have dissolved. Gradually whisk in the remaining 6 tablespoons vegetable oil. Whisk in some of the juices that have accumulated on the scallop plate until the dressing reaches the desired consistency.

4. Peel and section the grapefruit; cut each section in half. Tear the lettuce into bite-size pieces (about 4 cups). Halve, seed, peel, and slice the avocado; thinly slice the chives crosswise (about 1 tablespoon). Toss the lettuce with 1/3 cup dressing.

5. Mound the lettuce on each of 4 plates and top with the scallops, avocado, and grapefruit. Sprinkle with the chives and sunflower seeds and drizzle with the remaining grapefruit dressing.