Preheat the oven to 425°F (220°C).
Place the trimmed asparagus on a baking sheet. Drizzle with 1 tablespoon olive oil, sprinkle with 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper. Toss to coat evenly. Roast in the oven for 15 minutes until tender and slightly crisp.
While the asparagus roasts, place the peeled and chopped potatoes and smashed garlic cloves in a large pot. Cover with cold water and add a pinch of salt. Bring to a boil over high heat, then reduce to a simmer and cook until potatoes are tender, about 15 minutes.
Drain the potatoes and garlic well and return them to the pot. Add the butter, milk, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper. Mash until smooth and creamy. Keep warm.
Pat the salmon fillets dry with paper towels. Season both sides with 1 teaspoon kosher salt and 1/2 teaspoon black pepper.
Heat 2 tablespoons olive oil in a large non-stick skillet over medium-high heat until shimmering.
Place the salmon fillets skin-side down in the skillet. Cook without moving for 5-6 minutes until the skin is crisp and the salmon is cooked about two-thirds of the way through.
Carefully flip the salmon fillets and cook for an additional 2-3 minutes until the salmon is cooked through but still moist inside.
Remove the salmon from the skillet and let rest for 2 minutes.
Divide the garlic mashed potatoes and roasted asparagus evenly among four plates. Place one salmon fillet on each plate and serve immediately.