In a microwave-safe bowl, combine the sweet rice flour, water, and granulated sugar. Mix until smooth and well combined.

Cover the bowl with plastic wrap, leaving a small vent for steam to escape. Microwave on high for 1 minute. Stir the mixture, then cover and microwave for another minute. Stir again, then microwave for an additional 30 seconds until the mixture is thick and sticky.

Dust a clean surface with cornstarch. Carefully transfer the mochi mixture onto the surface. Allow it to cool slightly, then dust your hands with cornstarch and knead the mochi for a minute until it becomes smooth.

Divide the mochi into 8 equal pieces. Flatten each piece into a small disc. If using, place a small spoonful of red bean paste in the center of each disc, then add a strawberry half on top. Fold the edges of the mochi over the filling and pinch to seal.

Roll each filled mochi ball in cornstarch to prevent sticking. Place them on a plate and let them sit for about 30 minutes to firm up.