It sounds fancy, but it comes together quickly!
In a small bowl, mix 1 tbsp. parsley, the tarragon, lime zest, and garlic. Set the tarragon gremolata aside.
In a medium bowl, whisk 1 tbsp. oil, the capers, vinegar, sugar, and the remaining 1 tbsp. parsley; season with salt and pepper. Toss with the sliced fennel and arugula. Season the salad and divide among plates.
In a large skillet, heat the remaining 2 tbsp. oil and the butter over medium-high. Pat the scallops dry; season. Cook the scallops until opaque in the centers, 1 to 2 minutes per side.
Divide the scallops among the plates. Drizzle with the butter in the skillet. Top with the tarragon gremolata and fennel fronds. Serve immediately.