Chill a plate. Before you begin, place a small plate or saucer in the freezer. You’ll use it later to test whether the jam has finished cooking.
In a small saucepan, combine the strawberries, sugar, lemon juice, and salt. Stir to combine.
Let the mixture rest for 5 to 10 minutes so the strawberries can release their juices.
Place the saucepan over medium heat and bring the mixture to a gentle simmer, stirring frequently.
As the strawberries soften, mash them with a potato masher or the back of a wooden spoon until you reach your desired consistency.
Continue simmering one a lower heat for 30 minutes to an hour, stirring often, until the jam has reduced and thickened.
Test the jam. Remove the chilled plate from the freezer and spoon about 1 teaspoon of jam onto it. Return the plate to the freezer for 1 minute, then push the edge of the jam with your finger.
If the surface wrinkles and the jam holds its shape without quickly running back together, it’s ready.
If it still looks thin and flows easily, continue cooking for another 2 to 3 minutes, then repeat the test.
Remove the jam from the heat. It will continue to thicken slightly as it cools.
Let cool for about 15 minutes, then transfer to a clean 8-ounce jar. Cool completely before covering with a lid and refrigerating.