First, prepare your preserving pot. Wash jars, lids and bands in hot, soapy water. Heat jars in simmering water. Set lids and bands aside.
Combine the crushed strawberries, lemon juice, vanilla and sugar in a large saucepan. Bring to a boil over medium heat, stirring constantly to dissolve sugar. Boil, stirring frequently, until mixture thickens and gels (approximately 15 minutes). To test if the jam is ready, spoon a little onto a small plate and wait 30 seconds. Drag your finger through the jam and if the remaining jam doesn't "bleed" back, the jam is ready. Remove the jam from the heat and skim off any foam that may have risen to the top.
Ladle the hot jam into hot jars, one at a time, leaving 1/4 inch (0.5cm) headspace. Wipe the rim of the jar and center on the lids. Apply the bands and adjust to fingertip tight.
Process the filled jars in boiling water for 10 minutes. Remove pot lid. Wait 5 minutes, then carefully remove jars, cool and store up to 1 year in the pantry.
Notes
This recipe is originally from the BallĀ® fresh preserving website. It is recommended to follow their recipes when you preserve to ensure a safe end result.