1. Preheat the grill to medium high and the oven to 350°F.
2. Drizzle the cod fillets on both sides with 1 tablespoon of olive oil and season with salt and pepper. Place the fillets on a baking sheet and sprinkle with chives and tarragon. Place the baking sheet in the oven and roast until the fish is just opaque in the center, about 10 to 14 minutes, depending on the thickness of the fillets.
3. Meanwhile, peel and slice the onions 1/2" thick. Rub the onion slices (trying to keep them intact) with the olive oil and sprinkle with salt and pepper.
3. Grill the onions until they are lightly charred and cooked through, about 3 to 4 minutes per side. Set aside to cool.
4. In a medium sized bowl, place the diced tomatoes and season with salt and pepper. Stir in the balsamic vinegar. When the onions are cool enough to handle, chop roughly and combine them with the tomato mixture.
5. Serve each roasted cod fillet, on a warmed plate topped with about 1/4 cup of the tomato-onion relish.Serving Size: 1 fish fillet with 1/4 cup relish