Fish Pie with Mashed Potato Topping

Start by submerging the cubed potatoes into a large pot filled with salted water, then bring the mixture to a rolling boil. Once boiling, decrease the heat to a lower medium setting and let simmer until the potatoes are tender enough, approximately 15 minutes. Drain excess water.

Transfer the cooked potato cubes to a bowl on a stand mixer, blending them gently. Add cubed butter and milk while mixing until the potatoes reach your desired consistency. If needed, finish blending by hand to avoid overmixing and resulting in gluey potatoes. Season with salt and pepper.

Meanwhile, prepare the oven by preheating it to 400 degrees F (200 degrees C). During this time, heat butter and olive oil in a large skillet over medium heat. Add sliced onion and thyme and cook until the onions are soft and lightly browned, around 5 minutes. Deglaze with vermouth and cook until the alcohol has evaporated, approximately 2 minutes. Whisk in flour to create a roux, then slowly whisk in stock and milk, bringing the mixture to a gentle simmer. Let it cook for 1 to 2 minutes or until the sauce reaches your desired thickness, but feel free to thin it out with additional stock if needed. Season the sauce with salt and pepper if desired. Remove from heat and stir in half-and-half and chopped parsley.

Arrange the salmon, cod pieces, and shrimp at the bottom of a 2-quart baking dish. Sprinkle them lightly with salt and pepper, then drizzle lemon juice over the top of the seafood.

Pour the prepared sauce over the seafood and gently combine it with the fish. Spread the cooked potatoes evenly over the top, fluffing them with a fork before sprinkling Cheddar cheese over the potatoes.

Bake in the preheated oven for 10 minutes. Decrease the heat to 350 degrees F (175 degrees C) and bake until the potatoes are evenly browned all over, the seafood is cooked through, and the pie is bubbly and piping hot, approximately 20 more minutes.

Remove from the oven and allow it to rest for 10 minutes before serving.