Preheat your oven to 400°F and line a small baking sheet with parchment paper for easy cleanup.
Halve the baby potatoes and toss them in a bowl with 0.5 tsp of the olive oil, the dried rosemary, and a pinch of the sea salt.
Spread the potatoes on the baking sheet and roast for 20 to 25 minutes until they are golden brown and tender.
While the potatoes roast, pat the salmon fillet completely dry with a paper towel and season both sides with the remaining salt and black pepper.
Heat the remaining 0.5 tsp of olive oil in a stainless steel or cast iron skillet over medium-high heat until the oil shimmers.
Place the salmon skin-side down in the hot pan and sear for 4 to 5 minutes without moving it to ensure the skin becomes perfectly crispy.
Carefully flip the salmon and cook for an additional 2 to 3 minutes until the flesh is opaque and flakes easily with a fork.
In a small mixing bowl, whisk together the lemon juice and Dijon mustard to create a simple, clean vinaigrette.
Place the arugula in a bowl, toss with the prepared dressing, and serve immediately alongside the pan-seared salmon and roasted potatoes.