Place a small saute pan over medium heat. Add the fennel seeds and toast for 1 minute. Place both juices and fennel seeds in a medium saucepan and cook over high heat until reduced to 1 cup. Remove from heat and whisk in mustard and honey. Heat saute pan or grill pan over high heat. Season the fillets on both sides with salt and pepper and brush 1 side of each piece of fish with the glaze. Cook glaze-side down until golden brown, about 2 minutes. Turn over, brush with more of the glaze and continue cooking for 1 to 2 minutes for medium-rare doneness.
Fennel and Blood Orange Relish:
Whisk together the sherry, oil and salt and pepper in a medium bowl. Add the orange segments and fennel and mix to combine. Season with salt and pepper, to taste and fold in the parsley. Let sit at room temperature for 30 minutes before serving.
Heat oil in a large saute pan. Add the garlic and cook over medium-high heat for 1 minute. Toss in the watercress and cook just until it wilts. Add the vinegar, toss and serve.