In a small bowl, add 1 teaspoon of water, the cornstarch, and the lemon juice, stir until smooth.

In a small saucepan, add the raspberries, the remaining water, and the sugar. Place on the stovetop over medium-low heat, stir to combine. Crush the raspberries with the back of a spoon until broken up. When the mixture comes to a simmer, add the cornstarch mixture. Simmer for 3 minutes, stirring constantly.

Strain the mixture with a fine-mesh strainer over a medium-sized bowl. Press down, the sauce will separate from the raspberry seeds. Keep pressing the mixture through the strainer until only the seeds remain, scrape off any leftover sauce from the bottom of the strainer with a clean spoon.

Cool and cover with plastic wrap, store in an airtight container in the fridge until ready to use or use immediately. Use within 10 days.