Preheat the oven to 450 degrees. Toss potatoes, oil, and salt on a sheet pan. Add water directly to the pan.
Roast potatoes for 30 minutes. I recommend stirring every 10-15 minutes to prevent sticking. During this time the water will evaporate / steam the potatoes, and they will get beautifully golden at the same time. (Some sticking is normal and okay – just use a good spatula to firmly get under the potatoes and lift them to keep them all moving around and roasty toasty.)
Move potatoes to the side. Add the salmon filet in one piece to the center of the sheet pan, skin side down. Season with ½ teaspoon salt. Mix the mayo and Dijon in a small dish; spread over the top to cover. Bake for 6 minutes. Then broil close to the top of the oven for 2-3 more minutes to get browning on the salmon. You want to bake it until it flakes apart easily – see notes.
Mix the melted butter, grated garlic, and dill; spoon over the hot potatoes. Sprinkle the salmon with dill and red pepper flakes.
Squeeze lemon over the whole thing. Flake big chunks of salmon off to serve with the potatoes. Unreal.
