Combine raspberries and sugar in a sauce pan and bring to a boil. Use an immersion blender to make the filling smooth (or leave chunky if you prefer).
Combine your cornstarch and water to make a slurry. Pour it into the raspberry mixture and let it simmer until mixture is clear and thickened. Remove from heat, stir in lemon juice and zest and let cool before using.
Store leftovers in the fridge for up to a week or freeze for 6 months.
Notes
The most delicious lemon raspberry cake recipe ever! Lemon cake full of flavor and filled with fresh raspberry filling