If using sheet gelatin, soak the sheets in cold water for 15 minutes.
Puree the strawberries in a food processor. Drain through a fine strainer to eliminate the seeds. Heat the lemon juice and dissolve the powdered gelatin into it. Or, if using sheet gelatin, drain and dissolve the softened sheets into the warm lemon juice.
Blend the lemon juice into the pureed strawberries, add the powdered sugar, and mix thoroughly. Cool the mixture completely until it thickens.
Whip the cream and gently fold into the strawberry mixture. Pour into a lightly buttered mold and refrigerate for at least 3 hours.
Remove from mold, and garnish with cleaned and sweetened sliced strawberries.