To prepare the pickled pearl onions, mix water, vinegar, sugar, salt, garlic, mustard seed, peppercorns, ginger and jalapeño pepper in a medium saucepan. Cook over medium heat, stirring occasionally until it comes to a simmer and salt and sugar are dissolved. Cool.
Heat grapeseed or canola oil in a small sauté pan over medium-high heat. Add onions, and sauté until lightly caramelized. Add onions to the saucepan with the vinegar mixture. Let stand for 1 hour to pickle.
Strain the onions from the pickling juice, and place pickling juice in a small saucepan. Simmer for 15 to 20 minutes or until reduced to ½ cup. Remove from the heat, and set aside.
To prepare the apple-fennel salad, toss diced apples, fennel bulb, orange sections and raisins in a small bowl. Add 6 tablespoons of the reduced pickling juice and 6 teaspoons of the fennel fronds, and stir until combined; set aside.
Prepare a grill or preheat the broiler. Rub catfish fillets with olive oil, and season both sides with salt and pepper. Oil a grill rack or broiler pan rack. Grill or broil fillets about 4 inches from the heat source for 2 to 3 minutes on each side, or until fish flakes easily when tested with a fork.
Place a small mound of apple-fennel salad on each serving plate. Arrange pickled pearl onions on plates, and top with grilled catfish. Spoon remaining 2 tablespoons pickling juice around plates, and sprinkle with remaining 2 teaspoons fennel fronds.
Drizzle a little olive oil over the plates, and serve.