Place rack in top position of oven. Preheat broiler. Pat fish dry with paper towel and place on a foil-lined baking sheet. Season each fillet with salt and spread with mustard using the back of a spoon; sprinkle with dill and pepper.
Place lemon slices next to the fish in a single layer. Broil for 8 to 12 min., or until fish is opaque and lemons are lightly charred and juicy. Serve fish with lemon for squeezing.
High-heat cooking gives lemons a beautiful sweetness that pairs nicely with mild-flavoured sole.