Add cut strawberries, granulated sugar, and lemon juice to a large pot. Stir well. Place the pot over medium-high heat and bring the mixture to a boil.
Reduce the heat to medium-low and let it simmer. Stir frequently to prevent burning, and continue cooking for about 45 minutes until the jam thickens and reaches 220°F on a candy thermometer.
Remove the pot from the heat. If needed, skim off any foam from the top. Carefully ladle the hot jam into sterilized jars, leaving a little space at the top.