Preheat the oven to 400 degrees F, ensuring one rack is positioned in the center of the oven & a second rack is positioned in the lower third of the oven. Line 2 large baking sheets with parchment paper or foil for easy clean up. Gather & prep all ingredients.
Place the potatoes on one of the prepared sheet pans. Drizzle 2 tablespoons of olive oil over top & season with S&P as desired. Toss to coat the potatoes well. Transfer the sheet pan to the center rack of the oven & roast 20-25 minutes, until fork-tender.
Place the trimmed asparagus spears on the second prepared sheet pan. Drizzle with 1 tablespoon of the olive oil & season with the garlic powder and S&P as desired. Toss to coat the asparagus well, then arrange it on one half of the sheet pan. Season the salmon fillets with S&P as desired, then arrange on the second half of the sheet pan alongside the seasoned asparagus. Sprinkle the fresh dill over top & finish each fillet with 2 slices of lemon.
Once the potatoes are tender, remove the pan from the oven & set it on a sturdy work surface. Use the bottom of a thick glass or measuring cup to carefully smash each potato to ½-inch thickness – for best results, try to keep each potato in one piece as you smash down. Gently flip the smashed potatoes & drizzle the sheet pan with the remaining 1 Tbsp of olive oil.
Transfer the smashed potatoes to the lower rack of the oven & the sheet pan salmon and asparagus to the center rack of the oven. Bake for 15 minutes longer, until the potatoes are golden & crispy, the asparagus is tender, & the salmon is flaky.
Meanwhile, as the sheet pan salmon & crispy potatoes finish baking, prepare the lemon dill yogurt sauce. Combine all listed ingredients in a bowl. Season with S&P as desired. Stir to combine well. Set aside for serving, or store in an airtight container in the refrigerator for up to 5 days.