Wash and hull the strawberries thoroughly. Chop them into even pieces and measure out 4 cups; set aside.
Pour the chopped strawberries into a large, wide saucepan. Add the lemon juice and cook over medium heat, stirring occasionally, until the strawberries start releasing their juices and become soft, about 5 minutes. You should see a fragrant aroma and the berries breaking down slightly.
Gradually stir in the sugar and continue cooking, stirring constantly, until the mixture comes to a gentle boil. Continue boiling for 5 minutes, ensuring the sugar dissolves completely and the mixture thickens slightly.
Sprinkle the pectin evenly over the boiling mixture while stirring vigorously. Increase heat to medium-high and bring the mixture to a full, rolling boil that cannot be stirred down, about 1-2 minutes. You will notice the jam thickening and bubbling vigorously.
Remove the saucepan from heat and use a spoon to skim off any foam on the surface. Ladle the hot jam into sterilized jars, leaving about 1/4 inch headspace. Seal the jars tightly and allow them to cool to room temperature, during which the jam will set further.
Store the sealed jars in a cool, dark place for up to a month, or refrigerate for longer storage. Once opened, keep refrigerated and enjoy on toast, scones, or desserts.
