Combine all of the berries in a medium saucepan and stir gently to combine.
Add the lemon juice and sugar to the berries in the sauce pan. Bring to a gentle boil over medium heat and cook until the fruit is syrupy, about 5 minutes.
Transfer the hot berry mixture to a blender and purée until smooth or use a handheld blender to puree berries in saucepan. Set a fine mesh strainer over a bowl. Pour the sauce into the strainer and use the back of a soup ladle and circular motions to force the sauce through the strainer and into the bowl. Discard the seeds that remain in the strainer. Refrigerate the berry sauce until cold or ready to serve.
Before serving, add the reserved berries to the sauce and stir to combine. If the sauce seems too thick, add a few tablespoons of water, a little at a time, until the desired consistency is reached
Notes
Freezer-Friendly Instructions: The sauce can be frozen in an airtight container for up to 3 months.