Wash and prepare your fruit. If using strawberries, remove the stems and chop into small pieces. If using other berries, you can leave them whole or chop them if desired.
In a large saucepan, combine the fruit, sugar of choice, and lemon juice. Stir well to combine.
Place the saucepan over medium heat and bring the mixture to a boil, stirring occasionally.
Once boiling, reduce the heat to low and let the mixture simmer for about 20-30 minutes, or until it thickens and reaches a jam-like consistency. Stir occasionally to prevent sticking.
To test if the jam is ready, place a small amount on a chilled plate and let it cool for a few seconds. If it wrinkles when you push it with your finger, it's done. If not, continue simmering for a few more minutes and test again. This is where you can add the cornstarch if needed/wanted.
Once the jam is ready, remove the saucepan from the heat and let it cool for a few minutes.
Transfer the jam into clean, sterilized jars, leaving a little bit of space at the top. Seal the jars tightly.
