Preheat the oven to 400°F (200°C).
Place the peeled and cut 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 15-20 minutes.
While the potatoes cook, prepare the asparagus by placing it 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 another baking sheet lined with parchment paper or foil. Drizzle each fillet with the remaining 1 tablespoon olive oil and season with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Top each fillet with 2 lemon slices.
Put both the asparagus and salmon in the oven. Roast the asparagus for 12-15 minutes until tender but still crisp. Roast the salmon for 12-15 minutes until the flesh is opaque and flakes easily with a fork.
Drain the cooked potatoes and garlic well and return them to the pot. Add the butter and mash until smooth.
Warm the milk slightly and gradually add it to the mashed potatoes, stirring until creamy. Season with additional salt and pepper to taste.
Divide the mashed potatoes, roasted asparagus, and salmon fillets evenly among four plates. Garnish the mashed potatoes with chopped fresh parsley.
Serve immediately with extra lemon wedges if desired.