Very thinly slice fennel lengthwise; submerge slices in a bowl of ice water. Using a sharp paring knife, cut peel and white pith from oranges, then cut each section of fruit away from membranes, cutting as close to membranes as you can, and place sections in a bowl; squeeze juices from membranes into a second bowl. Slice oranges widthwise into ¼-inch slices. Remove white pith from 1 piece of peel, then thinly slice peel; set aside.

In a large skillet, heat 1 tablespoon oil over medium-high heat. Add swordfish and onion; season with salt and pepper and cook, stirring occasionally, for 2 minutes. Add orange juice and cook until fish is cooked through, about 1 minute more. Strain cooking liquid through a fine mesh strainer into a bowl; set swordfish aside and return liquid to skillet. Add raisins, capers and a pinch of the sliced orange peel to skillet, and heat over medium heat until warmed through. Remove pan from heat.

Drain fennel slices from water and pat dry thoroughly. Line a large plate with paper towels. Fill a medium skillet with enough oil to come up 1 inch. Over medium-high heat, heat the oil to 375º. Add half of the fennel and cook, turning once or twice, until golden, about 3 minutes. Using tongs or a slotted spoon, transfer fennel to paper towels to drain. Repeat with remaining fennel.

On serving plates, alternate layers of orange and swordfish. Spoon sauce over the top, pile fried fennel alongside, garnish with sliced orange peel and fronds, if using, and serve.