For the fish stock: Put the fish bones, onions, celery, fennel, bay leaves, peppercorns, white wine, and 3 cups water in a large pot over high heat. Bring the mixture to a boil, skim the impurities and reduce to a simmer for 20 to 30 minutes. Remove the stock from the heat and strain the stock into a large saute pan.
For the tarragon sauce: Mix the flour and butter together into a smooth paste (buerre manie), set aside. Put the fish stock in a large saute pan, bring to a boil, then reduce to a simmer for 5 minutes. Whisk in the flour-butter paste or buerre manie, ensuring there are no lumps. Once the liquid has reduced by 1/3, add the cream and whisk. Add the tarragon and season with salt, remove from the heat and set-aside until assembling the fisherman's pie.
For the potato gratin: Heat the milk and butter together. While the potatoes are still hot, press the potatoes through a ricer. Add the warmed milk and butter mixture to the potatoes and fold to incorporate. Add the egg yolks, mix and season with salt and pepper. Set aside for the fisherman's pie recipe.
For the vegetable filling: Heat the butter in a saute pan over medium heat. Saute the leeks until tender and translucent. Add the carrots and cook until just tender. Add the fish stock, peas, and season with salt and pepper. Remove from the heat and set aside.
For the fisherman's pie: Preheat the oven to 375 degrees F. Put the oven-proof dishes on a tray. Ladle equal amounts of 1/2 the tarragon sauce on the bottom of each dish. Put the catfish fillets in the center of the dishes. Divide the shrimp equally among the dishes and season with salt and pepper. Divide the vegetable filling evenly among the dishes and place over the fish and shrimp, then ladle equal amounts of tarragon sauce over. Top with equal amounts potato gratin and spread evenly to cover the filling. Bake for 15 minutes, then broil until potatoes are golden brown and remove from heat. Serve with Asparagus Salad and Lemon Vinaigrette.
For Asparagus Salad: Put the red onions, lemon zest and salt in a bowl, let sit 5 minutes. Rinse the onions and lemon zest and pat dry. Add the lemon juice, rinsed onions and lemon zest to a large bowl and toss. Using a vegetable peeler, peel the asparagus stalks into ribbons, coarsely chop the remaining center pieces and add to bowl with the onions. Add the lettuce, parsley, and olive oil and serve immediately.