Preheat the oven to 400 degrees F (200 degrees C). Cut a small slit in each potato and place onto a baking sheet.
Bake in the preheated oven until potatoes are easily pierced with a fork, about 1 hour 10 minutes.
While the potatoes are baking, melt 4 tablespoons butter in a skillet over medium heat; stir in onion. Cook and stir until onion has softened and turned translucent, about 5 minutes. Mix in Cheddar cheese, cream, mustard, and lemon juice. Season with salt and pepper. Cook, stirring frequently, until sauce is warm and Cheddar has melted, 3 to 5 minutes. Remove from heat.
Place white fish and shrimp in a 3 1/2-quart oven-proof baking dish. Pour cheese sauce over the fish; stir until well combined.
Scoop the flesh out of the baked potatoes and place in a large bowl; discard the skins. Add 4 tablespoons butter and season with salt and pepper. Mash with a fork until butter has melted and potatoes are smooth. Spread the mashed potatoes evenly over the top of the fish.
Bake until sauce is bubbling and potato topping is golden brown, about 55 minutes.