Prepare the strawberries: Wash the strawberries first in water in which baking soda is added and then thoroughly under cold running water. Remove the stems and hulls, then slice them into halves or quarters, depending on their size.
Sterilize the jars: Wash the jars and lids in hot, soapy water, then rinse them well. You can also sterilize them by boiling them in water for 10 minutes or running them through a dishwasher cycle.
Cook the strawberries: Place the sliced strawberries in a large, heavy-bottomed pot. Add the sugar, lemon juice, and lemon zest (if using). Stir well to combine.
Cook over medium heat: Bring the strawberry mixture to a gentle boil over medium heat, stirring occasionally to prevent sticking or burning. Skim off any foam that rises to the surface.
Simmer: Once the mixture reaches a boil, reduce the heat to low and let it simmer gently. Cook for about 30-40 minutes, or until the strawberries have softened and the mixture has thickened to your desired consistency. Stir occasionally to prevent sticking.
Test for doneness: To check if the preserves are ready, place a small amount on a chilled plate and let it cool for a minute. If it thickens and wrinkles slightly when you push it with your finger, it's done. If not, continue cooking for a few more minutes and test again.
Fill the jars: Once the preserves have reached the desired consistency, carefully ladle them into the sterilized jars, leaving about 1/4 inch of headspace at the top. Wipe the rims of the jars with a clean, damp cloth to remove any spills or drips.
Seal the jars: Place the lids on the jars and screw them on tightly. If you're using metal lids, you can process the jars in a boiling water bath for 10-15 minutes to ensure proper sealing. If you're using screw-on lids with built-in seals, this step may not be necessary, but it can help ensure a good seal.
Cool and store: Allow the jars to cool completely at room temperature before storing them in a cool, dark place. Properly sealed jars can be stored for up to a year. Once opened, store the preserves in the refrigerator and use within a few weeks.