Combine vinegar, sugar, tarragon, garlic, salt, and pepper in electric blender. With blender running, gradually blend in oil. Makes 1-5/8 cups dressing.
Combine rice, turmeric, and saffron until evenly mixed. Mix in half of the prepared Castilian dressing. Chill.
Cut onions into thick wedges, rub with oil, and spread out in pan. Roast at 400 degrees for 30 minutes or until roasted and tender; peel and trim off root.
For each serving, line plate with 1 cup lettuce. Top with 1/2 cup saffron rice, then 4 grilled prawns, 1/4 cup tomato, 1/8 cup cucumber, 1/8 cup green bell pepper, 3 artichoke quarters, and 2 or 3 wedges roasted onions.
Drizzle with 1-1/2 tablespoons Castilian dressing.