Lightly grease eight 4-oz. ramekins or cups with nonstick cooking spray.

Puree strawberries and 1/2 cup sugar in a blender until very smooth.

Press the strawberry puree through a sieve into a bowl to remove seeds; discard seeds.

Stir in vanilla extract into the sieved strawberry puree.

Remove 1 cup of the strawberry puree; reserve the remaining puree for sauce.

Sprinkle unflavored gelatin over 1/4 cup water in a small custard cup; let stand 5 minutes to soften.

Place the custard cup in a small skillet of simmering water; stir until gelatin is completely dissolved.

Simmer milk in a saucepan.

Remove the milk from heat.

Transfer the milk to a 1-qt. glass measure.

Stir in the reserved 1 cup strawberry puree, 1 tbsp sugar, and the heavy whipping cream until well blended.

Pour the mixture into the prepared ramekins.

Chill, covered, until firm, about 4 hours or up to 2 days.

To unmold, run a small knife around the inside of each ramekin.

Invert onto plates, gently shaking to remove.

Spoon some of the reserved strawberry sauce onto the panna cotta.

Garnish with sliced strawberries.