1. Preheat oven to 400° F. Line a large baking sheet with parchment for easier cleanup. Mix the olive oil and butter or ghee together, gently melting the butter/ghee first if necessary.
2. Drizzle the potatoes with one tablespoon of the olive oil-ghee mixture and generously season with salt and pepper. Spread the potatoes over half of the baking sheet and roast for 20 minutes.
3. While the potatoes roast, make the sauce and prep the salmon and asparagus. Combine the sauce ingredients in a small bowl and whisk until smooth, starting with one teaspoon of horseradish and adding more to taste.
4. Pat the salmon fillets dry and drizzle each with a little of the olive oil-ghee mixture and spread it evenly over all sides of the fish. Generously season with salt and pepper. Drizzle the remaining olive oil-ghee over the asparagus and season with salt and pepper.
5. Once the potatoes have roasted for 20 minutes, toss them and move them all back to one side of the pan. Place the salmon, skin-side down, on the other half of the baking pan and place the lemon slices over the salmon. Spread the asparagus around the salmon. Roast for 10 minutes and then check for doneness; if not done, check salmon every minute or two until just cooked through and the potatoes are fork tender.
6. To serve, flake the salmon from the skin (discarding the skin) onto a serving platter and gently toss it with the asparagus and potatoes. Drizzle with the horseradish sauce and add a dusting of smoked paprika and fresh dill just before serving, if desired.
Recommended Wine Style: Sauvignon Blanc