Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Dice the red potatoes into small 1/2-inch cubes to ensure they cook quickly and evenly.
In a bowl, toss the potatoes with the olive oil and half of the sea salt and black pepper.
Spread the potatoes on one side of the baking sheet and roast for 15 minutes.
While potatoes roast, finely chop the fresh parsley and dill, then mix them in a small bowl with the Dijon mustard.
Pat the salmon fillet dry with a paper towel and season with the remaining salt and pepper.
Spread the herb-mustard mixture evenly over the top of the salmon fillet.
Remove the baking sheet from the oven, toss the potatoes, and place the salmon on the other side of the sheet.
Return the tray to the oven and roast for another 10-12 minutes until the salmon flakes easily with a fork and potatoes are golden.
Squeeze fresh lemon juice over the salmon before serving.