Preheat the oven to 400°F (200°C).
Place the potatoes and garlic cloves in a large pot and cover with cold water. Add 1/2 teaspoon salt. Bring to a boil over high heat, then reduce to medium and simmer until potatoes are tender, about 15 minutes.
While the potatoes cook, place the asparagus on a baking sheet. Drizzle with 1 tablespoon olive oil and sprinkle with 1/2 teaspoon 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 with 1 tablespoon olive oil and season with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Place a lemon slice on top of each fillet.
Put both the asparagus and salmon in the oven. Roast the asparagus for 15 minutes until tender and slightly crisp. Roast the salmon for 12-15 minutes until cooked through and flaky.
Drain the potatoes and garlic and return them to the pot. Add the butter and mash until smooth. Gradually add the milk while mashing until creamy. Season with salt and pepper to taste.
To serve, divide the garlic mashed potatoes among four plates. Arrange the roasted asparagus alongside and place a salmon fillet on each plate. Garnish with chopped fresh parsley.