How to Make It
In a small bowl, whisk the mayonnaise, cornichons, red onion, tarragon, capers and mustard. Season with salt and pepper.
In a large skillet, arrange the pancetta slices in a single layer. Place a flat pot lid, slightly smaller than the skillet, directly on the pancetta to weight it down. Cook over moderate heat, turning once, until the pancetta is crisp, about 6 minutes. Transfer the pancetta to paper towels and wipe out the skillet.
In the skillet, melt the butter in the olive oil. Season the soft-shell crabs with salt and pepper and dust them generously with flour. Add the crabs to the skillet and cook over high heat, turning once, until crisp, about 7 minutes.
Spread the remoulade on the cut sides of the buns. Arrange the pancetta on the bottoms and top with the arugula, tomato slices and crabs. Close the sandwiches, cut them in half and serve.