Wash strawberries only when you are ready to use them. Do not remove the stems before washing, as the strawberries will absorb extra water.
After you wash them under cold water, let them drain in a colander for a few minutes. Check and discard any mushy or moldy berries.
Cut and discard the stems, and slice the strawberries into quarters or dice them.
Add them to a large, heavy-bottomed saucepan and mash using a potato masher. Mash them as much as you want, depending on whether you prefer the jam chunkier or smoother.
Add sugar, lemon juice, and lemon zest and let them macerate for about 20-30 minutes. The strawberries will release more juice.
Place the saucepan over low to medium heat and stir until the sugar is completely dissolved. Then, increase the heat to medium-high and bring the mixture to a rolling boil.
Once boiling, reduce the heat to medium and let it simmer, stirring occasionally, for about 15 to 20 minutes. If needed, continue simmering until the mixture becomes bubbly and thick.
If you would like your jam to be thicker, mix two tablespoons of cornstarch with four tablespoons of water in a cup until it is fully dissolved. Then, add this mixture to the jam and stir well. Simmer for a few minutes. If necessary, you can repeat this process, but remember that the jam will continue to thicken as it cools.
Pour the slightly cooled jam into jars and allow it to cool to room temperature. Seal the jars and store them in the refrigerator for up to two weeks or longer.
