Preheat the oven to 450°F (230°C).
Place the baby potatoes on a large sheet pan. Toss with olive oil and kosher salt until evenly coated.
Pour the water directly onto the sheet pan around the potatoes.
Roast for 30 minutes, stirring every 10–15 minutes to prevent sticking and ensure even browning.
Push the roasted potatoes to the sides of the pan.
Place the salmon fillet in the center, skin-side down.
Sprinkle the salmon with the remaining salt.
Stir together the Dijon mustard and mayonnaise, then spread evenly over the salmon.
Bake the Sheet Pan Salmon with Dill Potatoes for 6 minutes.
Switch the oven to broil and cook for another 2–3 minutes, or until the salmon is lightly browned and flakes easily with a fork.
Meanwhile, combine melted butter, grated garlic, and dill.
Spoon the garlic dill butter over the hot potatoes.
Sprinkle fresh dill and red pepper flakes over the salmon.
Finish with fresh lemon juice and serve immediately.