Prepare the peach filling: In a small saucepan over medium heat, combine diced peaches and 3 tablespoons sugar. Cook for 5-7 minutes, stirring occasionally, until peaches soften and release juice. Mash gently with a fork, then simmer until thickened to a jam-like consistency. Transfer to a bowl and refrigerate until completely cool.

Make the mochi dough: In a microwave-safe bowl, whisk together sweet rice flour, 1/4 cup sugar, and water until completely smooth with no lumps.

Cover the bowl tightly with plastic wrap. Microwave on high for 2 minutes, then carefully remove and stir vigorously with a wet spatula. The mixture will be partially cooked and sticky.

Re-cover and microwave for another 1 minute until the dough becomes translucent and pulls away from the bowl sides. It should look glossy and stretch when pulled.

Dust a clean work surface generously with potato starch. Turn the hot mochi dough onto the starch and dust the top heavily. Let cool for 3-4 minutes until touchable but still warm and pliable.

Divide the dough into 8 equal portions using a bench scraper or knife. Flatten each portion into a 3-inch circle in the palm of your hand, keeping edges slightly thicker than the center.

Place 1 tablespoon of cooled peach filling in the center of each circle. Gather the edges up and around the filling, pinching and twisting to seal completely. Roll gently between your palms to smooth.

Dust off excess potato starch with a soft brush. Place seam-side down on a parchment-lined plate and serve immediately, or store at room temperature.