Combine the strawberries, brown sugar and lemon juice in a large saucepan and bring to a simmer over medium-low heat, stirring, until the sugar dissolves. Partially cover with a lid and simmer, stirring occasionally, until the strawberries are softened and release their juices, about 10 minutes. Let cool.
Pour the strawberries into a blender and blend until smooth.
Press the sauce through a fine-mesh strainer into a storage container. The sauce will keep for up to 10 days in the refrigerator or up to 6 months in the freezer.