How to Make It
In a medium bowl, combine the potatoes, capers and
1 tablespoon of the olive oil, season with salt and pepper and toss to coat. On a heatproof plate that fits inside a large enameled cast-iron casserole, arrange the potatoes in a slightly overlapping layer. Place a small heatproof bowl upside down in the center of the casserole and place the plate with the potatoes on top. Add enough water to reach 1 inch up the side of the casserole. Bring the water to a simmer, cover the casserole and steam the potatoes over moderate heat until almost tender, about 15 minutes.
Rub the fish with the remaining 1 tablespoon of olive oil and season with salt and pepper. Arrange the fillets on top of the potatoes, skin side up, in a single layer and drizzle with 2 tablespoons of the grapefruit juice. Cover and steam until the potatoes are tender and the fish is just opaque throughout, about 8 minutes longer.
Meanwhile, in a small bowl, toss the avocado with the tarragon and the remaining 1 tablespoon of grapefruit juice; season with salt and pepper.
Transfer the potatoes and fish to plates. Top with any juices from the plate and the avocado salsa and serve.