In a small skillet, saute onion in oil until tender. Add couscous; saute 1-2 minutes longer or until couscous is lightly browned. Add water; bring to a boil. Reduce heat; cover and simmer for 8 minutes. Cool slightly.
In a large bowl, combine the couscous mixture, tuna, parsley, capers, olives, lemon zest, lemon juice, salt and pepper; set aside.
Pour vinaigrette over endive; toss to coat. On each of eight serving plates, layer a red tomato slice, a yellow tomato slice, 1/2 cup couscous mixture and 1/4 cup endive leaves.