Cook Potatoes: Add potatoes to a pot, cutting any larger potatoes to match the size of the majority. Add salted water to about 2 inches above potatoes and bring to a boil. Cover with a lid and vent slightly. Cook for about 10-15 minutes or until fork tender.
Prepare Vegetables + Sauce: Meanwhile, wash green beans and trim ends. Wash and finely chop dill. In a small bowl, stir together sour cream, horseradish, dill, half of the lemon juice, and a generous amount of black pepper to taste. Pat steelhead fillets dry using a paper towel. Season well with salt and pepper.
Cook Fish: When the potatoes are near done, start cooking the fish. Add 2 tbsp. (4 tbsp. for 4 servings) olive oil to a skillet over medium-high heat. Once hot, carefully add the steelhead fillets. Sear for 2-3 minutes on the first side of the fish. Meanwhile, add green beans to potatoes and cook for 2-3 minutes or until tender. When the first side of the fish has become golden and releases, turn it over. Turn off the heat immediately and move pan to a cold burner. The residual heat in the pan will finish the cooking the fish.
Finish + Serve: Strain potatoes and beans. Add 1 tbsp. (2 tbsp. for 4 servings) butter to the pot with garlic. When melted, add vegetables back to pot and toss to coat. To serve, divide the potatoes and green beans between plates. Lean steelhead on vegetables and drape horseradish cream onto the fish. Serve with remaining lemon if desired. Enjoy!