Add potatoes to a large 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 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.
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. Once hot, carefully add 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 from pan, turn it over. Turn off heat immediately and move pan to a cold burner. The residual heat in the pan will finish 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 potatoes and green beans between plates. Lean steelhead on vegetables and drape horseradish cream onto fish. Serve with remaining lemon if desired. Enjoy!