Heat a large, deep skillet over medium high heat and preheat oven to 425 degrees F.
Cut tops off the fennel bulb. Reserve a few tablespoons of fronds (lacy dill-like greens) and chop. Quarter the bulb, then cut into each quarter on an angle to remove the core. Thinly slice the fennel.
Add extra-virgin olive oil, fennel and onion to the pan and saute them, stirring frequently, 5 minutes. Season with salt. Add wine and reduce by half. Add potatoes to the pan in thin layers, completely covering the fennel and onions, working all the way to the edges of the pan. Pour the stock evenly over the pan. Season fish with salt and pepper. Arrange the fish on the potatoes. Bring the liquids to a bubble. Cover the pan and transfer to oven. Bake fish 15 minutes, until opaque, then uncover and bake for 2 or 3 minutes more. Transfer fish and vegetables to shallow bowls or dinner plates and garnish with parsley and fronds. Spoon juices down over the food. Serve with crusty bread for dipping.