Preheat your oven to 425°F (220°C). This high temperature will help achieve that perfect crispy texture on the potatoes and salmon.
In a mixing bowl, combine the 1 pound of cut potatoes with 4 tablespoons of melted salted butter, salt, and black pepper. Toss until the potatoes are well coated.
Spread the seasoned potatoes in a single layer on one side of the sheet pan. Roast them in the preheated oven for about 20 minutes, or until they start to soften and brown.
While the potatoes are roasting, prepare the glaze for the salmon. In another bowl, whisk together the 2 tablespoons of grainy Dijon mustard, 1 tablespoon of Dijon mustard, 2 tablespoons of lemon juice, chopped shallot, fresh thyme, cayenne pepper, and remaining 2 tablespoons of melted butter.
After the potatoes have roasted for 20 minutes, remove the sheet pan from the oven. Place the 2 pounds of salmon fillets on the empty side of the pan. Season the salmon with salt and black pepper, then generously brush the Dijon glaze over each fillet.
In a small bowl, mix the 1/2 cup of panko breadcrumbs with 3 tablespoons of sesame seeds and chopped garlic. Sprinkle this mixture over the salmon fillets for an added crunch.
Return the sheet pan to the oven and roast for an additional 12-15 minutes, or until the salmon is cooked through and flakes easily with a fork. The potatoes should be golden and crispy by this point.
In the last 2-3 minutes of roasting, add the arugula or shaved asparagus to the pan around the salmon and potatoes, allowing them to wilt slightly.
Once everything is cooked, remove the sheet pan from the oven. Sprinkle fresh chopped dill over the salmon and serve hot, enjoying the beautiful presentation and delightful flavors of your Sheet Pan Dijon Salmon and Panko Potatoes.