Prep and cook instructions are almost identical for all fish options.

• Pat the fish dry with a paper towel; season the salmon or halibut generously with salt and pepper, the trout lightly.

In a large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Working in batches if needed, add the fish (skin side down for the salmon and trout) and cook until lightly browned (and the salmon or trout skin is crisp), 4 to 5 minutes for salmon or halibut, 2 to 4 minutes for trout. Turn and cook until the flesh is opaque and flaky, 2 to 4 minutes for salmon or halibut, 1 to 2 minutes for trout. Transfer to a plate to cool slightly. Add more oil between batches if needed. Using a fork, flake the fish into bite-size pieces; if using salmon or trout, discard the skin if desired. Transfer the fish to a large bowl. Do not clean the pan.
While the fish cooks and cools, prepare the remaining ingredients.

• Rinse the black beans.
• Peel the watermelon radish and trim the ends; cut the radish in half lengthwise, then crosswise into thin half-moons.
• Remove the stem, ribs, and seeds from the bell pepper; cut the pepper lengthwise into ¼-inch-wide strips, then crosswise into enough ¼-inch pieces to measure ½ cup [1 cup].
• Trim the root end from the romaine; separate the leaves.

In the same pan used for the fish, if dry, add 1 teaspoon [2 tsp] oil. Warm over medium-high heat until hot but not smoking. Add the beans and cook, stirring occasionally, until warmed through, 2 to 3 minutes. Remove from the heat and season to taste with salt and pepper.
To the bowl with the fish, add the beans, watermelon radish, bell pepper, and half the green chermoula and gently toss to coat (set aside the remaining chermoula for serving). Season to taste with salt and pepper.

Transfer the romaine leaves to individual plates, stacking 2 or 3 leaves on top of one another. Fill the leaves with the fish and bean salad and serve the remaining chermoula on the side.