Rinse fish and pat dry; set aside. In a shallow dish, stir together the flour, salt, and pepper to taste. Dip each fillet in the flour mixture to coat.
In a skillet, heat oil over medium-high heat. Cook the fish in the hot oil for 2 to 3 minutes per side or until golden.
In a small bowl, whisk together the mayonnaise, mustard, and dill. Set aside.
Spread the dill mayonnaise evenly on the rolls. Place a fish fillet on each roll and top with tomato and onion slices