Pluck the stem off each strawberry and remove the tiny core with a paring knife. (Or, use a strawberry hulling tool if you have one.)
Chop the strawberries.
Zest the lemon and set it aside.
Put the strawberries, lemon juice, and sugar in a medium pot. Stir over low heat until the sugar is dissolved. This will take about 3 minutes.
Bring the mixture to a boil and continue to cook on medium-high heat for 10 minutes. It will get thick and bubbly. Take a potato masher and break down the strawberries in the pot.
Remove the jam from the heat and stir in the lemon zest. Let the mixture cool for 10 minutes before transferring it to a glass jar. It's now ready to serve or store in the fridge.