Step 1Cut a thin piece off the end of each lemon to make a flat base. Cut a 1/3-inch piece off the other end. Use a small, sharp paring knife and grapefruit spoon to scoop out the lemon flesh and make a clean shell. Freeze until solid, about 30 minutes.
Step 2Fill each with sorbet. Blot the cut edges of the rinds with a paper towel. Freeze until solid. (These can be kept a week ahead, covered airtight.)
Step 3Place a rack about 7 inches under the broiler element; heat the broiler.
Step 4Use a mixer to beat the egg whites, sugar and cream of tartar on medium-high speed until stiff peaks form, about 3 minutes. Spoon the meringue into a pastry bag fitted with star tip. Top each lemon with the meringue, dividing evenly. Make sure the meringue seals cut the edges of the lemon. Set the lemons on a baking sheet.
Step 5Broil until the meringue is singed, turning as necessary, about 30 to 60 seconds. Alternatively, use a propane torch. Freeze the lemons. (These can be kept frozen, loosely covered with plastic wrap, up to 5 days.)
Step 6To serve, let lemons rest at room temperature 20 minutes before serving.