Heat 1 tbsp olive oil in a large skillet over medium-high heat. Add shrimp, scallops, and fish, cooking 2–3 minutes per side until just opaque.
Reduce heat to medium, add chopped onion, minced garlic, and diced carrots. Cook 4 minutes, stirring occasionally until softened and fragrant.
Stir in heavy cream, frozen peas, and chopped parsley. Simmer gently for 3 minutes until sauce thickens.
Peel russets and cut into chunks. Boil in salted water for 15 minutes until fork-tender. Drain thoroughly.
Return potatoes to the pot, add butter, egg yolk, salt, and pepper. Mash gently until smooth.
Transfer the seafood filling into a greased 9-inch baking dish. Spoon mashed potatoes on top, smoothing edges to create a decorative crust.
Place in a preheated 400°F oven and bake for 20–25 minutes, until the crust is crisp and golden brown.
Remove from oven and let stand 5 minutes before serving.
