Adjust racks to middle and top positions and preheat oven to 450 degrees. Wash and dry all produce. Dice potatoes into 1/2-inch pieces; place in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Meanwhile, trim and discard woody bottom ends from asparagus. Finely chop chives. Zest and quarter lemon.

Toss asparagus on a baking sheet with a drizzle of olive oil, salt, and pepper. Pat trout dry with paper towels. Rub fillets all over with a drizzle of olive oil; season generously with salt and pepper. Rub flesh sides of fillets with Fry Seasoning; place on a separate, lightly oiled baking sheet.

Roast asparagus on middle rack and trout on top rack until asparagus is tender and lightly browned and trout is opaque and cooked through, 10-12 minutes.

Once potatoes are tender, drain and return to pot over low heat. Mash with 2 TBSP butter (4 TBSP for 4 servings), half the horseradish powder, and milk (add a few splashes at a time until smooth). Taste and add more horseradish powder if desired. Stir in half the chives (you’ll use the rest later). Season generously with salt and pepper. Turn off heat; keep covered until ready to serve.

Meanwhile, in a small bowl, combine stock concentrate, a big squeeze of lemon juice, half the lemon zest, and 1/4 cup water (1/3 cup for 4 servings); set aside. Heat a medium, dry pan over mediumhigh heat. Add almonds and cook, stirring, until golden brown, 2-4 minutes. Transfer to a plate. Pour stock mixture into same pan over medium-high heat. Bring to a boil and cook until slightly thickened, 2-3 minutes. Turn off heat.

Stir 2 TBSP butter (3 TBSP for 4 servings) into pan with sauce until melted, then stir in remaining chives. Taste and season with salt, pepper, and more lemon juice if desired. Toss asparagus with remaining lemon zest and as many toasted almonds as you like. Divide asparagus, potatoes, and trout between plates. Spoon sauce over trout. Serve with any remaining lemon wedges on the side.