If you're using frozen berries, allow them to thaw first. 

Purée the strawberries using a blender or potato masher.

Mix together the water, lemon juice, sugar, and puréed strawberries in a small saucepan.

Before heating, whisk the corn starch into the mixture. Whisking will ensure a lump-free sauce.

Bring the mixture to a boil on low to medium heat and simmer for about 3 minutes, or until your sauce has the desired thickness.

You can pour or spoon this delicious strawberry sauce on pancakes, waffles, ice cream, etc. I actually swirled most of mine into my cheesecake batter.