Add the rinsed raspberries, lemon peels and water to a saucepan. Place on medium heat and boil for 15 minutes.
After 15 minutes, use a wooden spoon to stir and mash the raspberries before carefully pouring the hot mixture into the strainer.
Carefully pour the strained raspberry liquid back into the saucepan, discarding the seeds, on medium heat.
Add the brown sugar and stir with a whisk or wooden spoon until dissolved.
Simmer on medium heat for another 15 minutes .
Remove ¼ cup of the raspberry liquid and add to a small bowl. Add the cornstarch and quickly stir with a fork to dissolve any lumps.
Add the cornstarch mixture back to the simmering raspberry liquid and quickly stir to incorporate. Cook for an additional 3 minutes to thicken then remove the saucepan from the heat.
Pour the hot raspberry jam in a heat safe container and allow to cool at room temperature before serving or storing.