How to Make It
In a large saucepan of boiling salted water, cook the shrimp until loosely curled and just cooked through, about 2 minutes. Drain well; transfer to a large bowl and let cool slightly.
In a small bowl, combine the grapefruit and lime juices, vinegar and oregano and season with salt and pepper.
Add 2 tablespoons of the citrus dressing to the shrimp, toss well and season with salt and pepper. Season the tomatoes and avocado slices with salt and pepper and arrange on a platter with the onion, alternating and overlapping the slices. Spoon the remaining citrus dressing over the salad and top with the shrimp. Drizzle olive oil over all and serve immediately.