Add fresh cranberries, fresh raspberries, brown sugar, and lemon juice to a medium saucepan over medium heat. Allow the mixture to cook down for 10-15 minutes, let the cranberries pop and reduce.
Then add a 1/2 cup water or cranberry juice and continue to cook for another 10 minutes, until the mixture thickens. Mix together cornstarch and 1 tbsp of water in a small bowl and add to the sauce.
Cook for 3-5 more minutes and remove from the heat. Add butter and stir until it has melted. Allow the cranberry raspberry sauce to cool. Serve cold or at room temperature.
Notes
Substitutions
If you don't have or can't find fresh raspberries, you can use raspberry jam. I recommend using about 1/4 cup. Simply stir it into the reduced cranberries (more info in the recipe card).
Use pure maple syrup or honey in place of sugar if you prefer natural sweeteners.
In place of cranberry juice, you can also use apple juice or orange juice.
You can use frozen cranberries and raspberries, however, you may need to cook the sauce a little longer as frozen fruit tends to release a bit more water.