Preheat the oven to 425°F. Line a baking sheet with parchment paper and place the whole salmon filet onto the baking sheet. Rub the salmon with the olive oil and season with salt and pepper. Place the lemon slices down the center of the salmon. Bake for 18 minutes.
While the salmon is baking, prepare the sauce. In a small mixing bowl, combine the mustard, mayonnaise, maple sugar, and lemon juice. Whisk all ingredients together until fully combined. Add the scallions and dill and stir to combine. Once the salmon is cooked, serve with the mustard dill sauce.