Step 1: Prepare the Watermelon Filling
1. Puree Watermelon: In a blender, puree the watermelon until smooth. Strain through a fine mesh sieve to remove the pulp if desired.
2. Sweeten: In a small saucepan, combine the watermelon puree, sugar, and lemon juice. Heat over medium heat, stirring until the sugar dissolves. Allow it to simmer for about 5-10 minutes until slightly thickened.
3. Cool: Remove from heat and let it cool. Once cooled, pour the mixture into a shallow dish and freeze for about 30 minutes until set.

Step 2: Make the Mochi Dough
1. Mix Ingredients: In a microwave-safe bowl, combine the sweet rice flour and granulated sugar. Gradually add the water, stirring until smooth.
2. Microwave: Cover the bowl with plastic wrap and microwave on high for 1 minute. Stir the mixture, cover again, and microwave for another minute. Repeat this process for a total of 3 minutes, stirring in between, until the mochi is thick and translucent.

Step 3: Shape the Mochi
1. Dust Surface: Dust a clean surface with cornstarch or potato starch to prevent sticking.
2. Roll Out Mochi: Transfer the hot mochi dough onto the dusted surface. Dust the top of the dough with more starch. Use a rolling pin to roll it out to about 1/4 inch thick.
3. Cut Out Circles: Use a round cookie cutter or a glass to cut out circles from the mochi dough.

Step 4: Assemble the Mochi
1. Fill with Watermelon: Take the chilled watermelon filling from the freezer and cut it into small squares. Place a piece of watermelon filling in the center of each mochi circle.
2. Seal the Mochi: Gently fold the edges of the mochi circle over the filling and pinch to seal. Dust with more cornstarch to prevent sticking.

Step 5: Serve
1. Chill: Place the finished mochi on a plate and chill in the refrigerator for about 15 minutes to firm up.
2. Enjoy: Serve chilled and enjoy your refreshing Watermelon Mochi!