Wash two 2-cup canning jars or other jars with tight-fitting lids very well, and allow to dry completely.
Combine the strawberries and sugar in a medium saucepan. Bring to a boil over medium-high heat, stirring frequently until the sugar has dissolved and the berries are starting to break down. Add the lemon juice, lower the heat to medium-low, and cook at a gentle simmer for about 25 to 30 minutes, stirring often, until the mixture looks like loose jam and has reached a temperature of 220 F measured with a candy thermometer (see Note).
As the jam cooks, you can crush the berries with a wooden spoon against the side of the pot as you go, or use a potato masher. Leave some chunks of berry in there for texture. Spoon the jam into the clean jars and allow to cool to room temperature.
Seal the jars and refrigerate.
Notes
If you don’t have a candy thermometer, that’s ok. You can just cook it until it starts to look like loose jam. It will thicken upon cooling.