Heat oven to 230°C. Rub the oil on the bottom of a rimmed baking sheet and place the salmon on top. Season with the coriander, fennel, 1 teaspoon salt, and ½ teaspoon pepper. Roast until just opaque but still slightly pink in the center, 16 to 20 minutes.
Meanwhile, combine the crème fraîche, cucumber, horseradish, ½ teaspoon salt, and ¼ teaspoon pepper in a small bowl.
Top the salmon with a little of the horseradish sauce and the tarragon. Serve warm or at room temperature, with additional horseradish sauce on the side.