Preheat oven to 425.
Slice lemon in half. Reserve half and slice the remaining half into 4 thin slices. Set aside.
Add potatoes to a large bowl and drizzle with 2 tbsp olive oil. Sprinkle with parmesan cheese and garlic powder. Add salt and pepper to taste. Toss to coat.
Arrange potatoes cut side down on a greased baking sheet and roast for 20-25 minutes or until fork tender.
Remove potatoes from oven and arrange salmon in the center of pan, fill in any holes with broccolini and drizzle one tbsp. olive oil over broccolini.
Sprinkle salt over entire baking sheet. Squeeze the juice of half a lemon over the salmon and add 1 tbsp garlic butter on top of each filet.
Return to oven and watch closely. Once butter has fully melted add a thin lemon slice to each salmon filet. Continue cooking until salmon is no longer translucent but still moist in the center, about 12 minutes.