Cook Strawberry Mixture: In a pot over low heat, combine the mashed strawberries, strawberry jelly powder, and water. Let the mixture heat until the strawberries dissolve and blend with the jelly, stirring occasionally.

Blend with Gelatin: Transfer the mixture to a blender and mix well. Then, add the gelatin powder and blend again until everything is fully combined.

Strain and Cool: Pour the mixture through a strainer to remove any lumps. Set it aside to cool down.

Combine with Yogurt: In a deep bowl, mix the yogurt and sugar. Then, add the cooled strawberry mixture and stir well.

Chill to Set: Pour the mousse into serving cups and place them in the refrigerator for about 5 hours, or until the mousse is fully set and firm.

Serve: Garnish the cups with fresh strawberry pieces and serve cold.