Preheat oven to 400 degrees. Place fish on a rimmed baking sheet; season with salt and pepper. Roast until cooked through, 15 to 20 minutes. Let cool completely; pat dry with paper towels. Break salmon into smaller pieces; flake with a fork.
In a large bowl, combine scallions, breadcrumbs, and parsley. Add mayonnaise, mustard, eggs, lemon juice, and salmon. Season again with salt and pepper, plus an additional flavoring if desired. Mix gently until ingredients just hold together.
Form mixture into 8 patties (each about 3 inches wide). Place on a foil-lined baking sheet. Refrigerate until cold and firm, 20 to 30 minutes.
Make Lemon-Herb Sauce: Whisk together mayonnaise, sour cream, fresh lemon juice, and chopped herbs. Season with coarse salt and ground pepper.
Heat broiler. Broil patties until lightly golden on top (without turning), 6 to 8 minutes. Serve with Lemon-Herb Sauce.
To freeze: Form salmon cakes, but do not broil. Place them on a baking sheet; freeze until firm. Wrap each cake in plastic; transfer to resealable freezer bags. Store up to 1 month. Defrost overnight in the refrigerator before broiling.