1. Cut tomatoes in half horizontally. Scoop out and discard seeds. Finely chop tomatoes; place in a large bowl. Stir in juice and oil; season with salt and pepper. Gently toss in salmon, avocado and chives.
2. Cut bottom off each head of endive; separate leaves. Arrange 24 large outer leaves on a platter. Reserve smaller inner leaves for another use. Fill lower half of leaves with a heaping tablespoon of salmon salad. Serve, garnished with additional chives if desired.