Preheat oven to 425F degrees. While oven comes to temp, wash your potatoes and cut them in half. The bigger ones can be cut in quarters to make sure they are all about the same size.
On a lined baking sheet greased with cooking spray, toss the potatoes, olive oil, lemon juice, and seasoning together so potatoes are completely covered with the mixture.
Spread potatoes in an even layer on the sheet and place on the upper rack of your oven. Bake for 15-20 minutes, or until the potatoes are tender when pierced with a fork. Cooking time will vary depending on how small you cut your potatoes.
While potatoes are cooking, prep your salmon. Cut into fillets (if needed). Evenly spread dijon mustard over all 4 fillets. Combine all seasoning together in a small bowl and sprinkle over the dijon on top of the fillets. Season very liberally to impart tons of flavor into the salmon.
When potatoes are done, remove sheet pan from oven. Push potatoes to one side. Add salmon down the middle, then asparagus on the opposite side of the sheet. Spray asparagus with cooking spray, then season with lemon juice, salt, and pepper.
Place sheet back into the oven on the upper rack and cook for 5-8 minutes, depending on the size and thickness of your salmon fillets. If you want to be particular, the salmon should reach an internal temp of 145F.
(Optional) Broil for the last minute or two for extra browned bits on top. Serve with extra lemon.