Preheat oven to 425F, optionally line a baking sheet with heavy duty aluminum foil for easier cleanup, spray with cooking spray, add the potatoes, evenly drizzle with 3 to 4 tablespoons olive oil, evenly season with salt and pepper, and toss with your hands to combine and evenly coat. Bake for about 15 minutes, or until potatoes are about 75% done.

While the potatoes are baking, to a small microwave-safe bowl, add the butter and heat on high power to melt, about 45 seconds. Add the lemon juice, Dijon, and stir to combine; set aside.

Remove the baking sheet from the oven, flip the potatoes to ensure even cooking, add the salmon fillets skin-side down, evenly drizzle with the remaining 2 to 3 tablespoons olive oil, and nestle the potatoes around the salmon.

Evenly drizzle about two-thirds of the lemon butter Dijon mixture over the salmon fillets. Evenly drizzle the remaining one-third over the potatoes. Some of the mixture will runoff the salmon fillets; this is normal and okay.

Evenly season the salmon with salt and pepper, to taste.

Bake for about 10 minutes, or until the salmon and potatoes are done. The salmon should flake easily with a fork and the potatoes should be fork tender.

Optional: For crispier salmon and potatoes, place the sheet pan under the broiler for 2 to 3 minutes, or just until as browned as desired; keep a close eye on your food while it's under the broiler because it can burn very quickly.

Optionally garnish with parsley and serve immediately.