Lettuce (I used combination of romaine and radicchio)
1Preheat broiled and line a large shallow baking pan with foil.
2Toss onion and half of the tomatoes with 2-½ tbsps oil, ½ tsp salt and ¼ tsp pepper in lined baking pan, then spread evenly in one half of pan.
3Season the dry mahi-mahi with ¼ tsp each of salt and pepper. Place skin side down next to tomato mixture in other half of baking pan.
4Drizzle fish with 1 tbsp oil and broil 4-6 inches from heat until vegetables are charred and fish is just cooked through, 12-15 minutes.
5Meanwhile, whisk together vinegar, tapenade, ¼ tsp pepper, and remaining 2-½ tbsps of olive oil.
6Alternating between romaine and radicchio, place six pieces of lettuce (3 romaine and 3 radicchio) on each of four plates, touching in center. In between each slice of lettuce, place a cherry tomato. Place a fish on each plate and evenly distribute tomato-onion mixture on top. Stir pan juices into vinaigrette and drizzle on top of the fish. Serve immediately.