Easy Fish Pie

Step 1
Preheat the oven to 400 degrees F (200 degrees C). Cut a small slit in each potato and place onto a baking sheet.

Step 2
Bake in the preheated oven until potatoes are easily pierced with a fork, about 1 hour and 10 minutes.

Step 3
When the potatoes are almost finished, melt 1/4 cup butter in a skillet over medium heat. Add onion; cook and stir until softened and translucent, about 5 minutes. Mix in Cheddar, 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 the heat.

Step 4
Place white fish and shrimp into a 3 1/2-quart oven-proof baking dish. Pour cheese sauce over fish and stir until well combined.

Step 5
Scoop flesh out of baked potatoes and place into a large bowl; discard the skins. Add remaining 1/4 cup butter; season with salt and pepper. Mash with a fork until butter has melted and potatoes are smooth. Spread mashed potatoes evenly over fish.

Step 6
Return to the oven and bake until sauce is bubbling and potato topping is golden brown, about 55 minutes.