Coat nonstick frypans with cooking spray. Sauté trout, skin-side down, over medium-high heat until golden and slightly crisp, about 4 minutes on each side, turning once. Place trout skin-side down in a shallow pan.
Sauté shallots in a nonstick pan over medium heat until softened. Stir in orange juice, mirin and fennel seeds. Pour mixture over trout. Cover and refrigerate at least 1 hour, or overnight.
Combine walnuts, sugar, salt, five-spice powder and egg whites in a medium bowl. Spread in a single layer on nonstick baking sheet; bake at 250°F about 45 minutes, or until golden and almost dry, stirring occasionally. Cool. Break apart any larger clumps.
Remove trout from marinade, reserving marinade. Remove and discard skin. Cut trout to make 12 pieces. Pour marinade into a saucepan; cook until reduced to 1/2 cup. Cool slightly before whisking in walnut oil to make a dressing.
Combine orange slices, fennel, and watercress in large bowl. Drizzle with dressing; toss to coat. For each serving, place 3 cups salad on a large plate; top with a piece of trout. Sprinkle with 1/4 cup walnuts. Garnish with 1 orange slice and a few fennel tops.