Cut the shrimp in half lengthwise, without cutting all the way through, then carefully open them into butterfly shrimp.
In a skillet over medium heat, heat the olive oil. Add the shallot and garlic, then cook for 2 minutes over medium-low heat until softened but not browned.
Remove from the heat and stir in the old-fashioned mustard, parsley, Aleppo pepper, lemon zest, lemon juice, and a pinch of salt. Mix well.
Reserve half of this mixture for serving.
Place the shrimp on a baking sheet lined with parchment paper. Brush with the remaining half of the marinade. Bake for 12 to 15 minutes, until the shrimp are pink and cooked through.
In a bowl, whisk together the old-fashioned mustard, mayonnaise, honey, lemon vinegar, and a drizzle of olive oil.
Arrange the arugula and fennel on a plate. Drizzle with the vinaigrette, then garnish with pomegranate arils and fresh dill.
Serve the grilled shrimp with the salad and drizzle the shrimp with the remaining oil-based marinade.
