Preheat oven to 450 degrees F (230 degrees C). Oil a baking dish.
Microwave potatoes in a microwave dish on High until just softened, about 5 minutes.
Whisk olive oil and sherry vinegar together in a bowl. Add smoked paprika and salt; whisk to blend. Stir in onions, red jalapeno, and cooked potatoes. Slide the fish fillets into the mixture, turning and coating them with the vinaigrette. Remove the fish from potato mixture.
Place potatoes in prepared baking dish. Nestle fish onto potatoes. Sprinkle with a pinch of salt, a dash of paprika, and a drizzle of olive oil.
Bake in center of preheated oven until just cooked through and fish flakes easily, about 15 minutes.
