Heat oven to 350ºF (176ºC) with rack set in middle position.
Meanwhile, zest lemon then cut lemon into wedges. In a small bowl, combine ¼ cup mayonnaise, 2 tablespoons chopped dill, 2 tablespoons minced shallot, ½ teaspoon lemon zest, ½ teaspoon pepper, and ¼ teaspoon kosher salt.
Coat baking sheet with nonstick spray. Arrange salmon, skin-side down, on sheet; pat dry with paper towels. Season salmon with ½ teaspoon each kosher salt and pepper.
Spread mayonnaise mixture evenly over salmon fillet(s).
Roast salmon until it easily flakes with a fork and a thermometer inserted in the center registers 125ºF, 15–20 minutes. (Or 7–10 minutes for thin, wild-caught fillet.)
Top with toasted breadcrumbs, optional. Serve with lemon wedges.