Place potato chunks in a pot, cover with cold water, and bring to a boil. Simmer for 15–20 minutes until fork-tender.

Drain well, return to pot, add 1/4 cup butter and 1/2 cup milk, and mash until smooth. Season with salt and pepper and keep warm.

Pat shrimp and salmon dry with paper towels. Rub both sides with Cajun seasoning, pressing gently so it adheres.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add shrimp in a single layer and cook for about 1–2 minutes per side until opaque and pink. Remove shrimp to a plate.

In the same skillet, add a little butter if needed and place salmon fillets skin-side down (if skin on). Cook for 3–4 minutes per side until the salmon flakes easily. Remove to the plate with the shrimp.

Reduce heat to medium. Melt 2 tablespoons butter in the skillet. Add 3 minced garlic cloves and cook for 30–45 seconds until fragrant, being careful not to brown.

Pour in 1 cup heavy cream and 1/4 cup chicken broth, whisk in 1 teaspoon Dijon mustard, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and 1/4 teaspoon smoked paprika. Simmer for 3–5 minutes, stirring, until slightly thickened.

Return shrimp and salmon to the skillet and spoon sauce over them. Let everything warm through for 1–2 minutes so flavors marry. Taste and adjust seasoning.

Bring a pot of salted water to a boil, blanch green beans for 2–3 minutes until bright green and slightly tender. Drain and immediately toss in a skillet with 2 tablespoons butter, 2 minced garlic cloves, and 1/4 teaspoon red pepper flakes if using. Sauté for 1–2 minutes until garlic is fragrant.

Spoon mashed potatoes onto plates, arrange salmon and shrimp on top, ladle garlic cream sauce over seafood, and add green beans alongside. Garnish with chopped parsley or lemon wedges if desired.