Preheat the oven to 425°F (220°C).
Place the peeled and cut potatoes and garlic cloves in a large pot and cover with cold water. Add 1/2 teaspoon kosher salt. Bring to a boil over high heat, then reduce to medium and simmer until potatoes are tender, about 15-20 minutes.
While the potatoes cook, place the trimmed asparagus on a baking sheet. Drizzle with 1 tablespoon olive oil and sprinkle with 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper. Toss to coat evenly.
Place the salmon fillets on a separate baking sheet lined with parchment paper. Drizzle the remaining 1 tablespoon olive oil over the salmon. Season each fillet with 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper.
Put both baking sheets (asparagus and salmon) in the oven. Roast the asparagus for 12-15 minutes until tender and slightly browned. Roast the salmon for 12-15 minutes until it flakes easily with a fork.
Drain the cooked potatoes and garlic and return them to the pot. Add the butter and mash with a potato masher until smooth.
Warm the milk in a small saucepan or microwave until hot but not boiling. Gradually add the warm milk to the mashed potatoes, stirring until creamy and smooth. Season with additional salt and pepper to taste.
Divide the garlic mashed potatoes and roasted asparagus evenly among four plates. Place one salmon fillet on each plate.
Sprinkle the chopped fresh parsley over the mashed potatoes and salmon for garnish. Serve immediately.