This is delicious when served with bread or crackers.
Pour the lemon juice into a small bowl. Whisk in the olive oil and season with salt and pepper. Add the shrimp and scallions and toss lightly. Let stand for 15 minutes to let the flavors blend. Add the tomato, avocado, and nasturtium leaves. Mound the mixture on lettuce leaves and garnish with the nasturtium blossoms.