Wash and clean the strawberries very well and remove the stems. Chop the smaller ones into quarts and the bigger ones into eights.
Wash well 2 small glass jars and their lids. Keep the jars in a warm place (or in the oven heated to 100 degrees C/ 200 degrees F). By warming them we are preventing breakage from the temperature difference, as they will be filled with hot jam.
In a large pan over medium heat combine chopped strawberries and sugar. With a knife split the vanilla bean lengthwise and scrape down its seeds. Add both the seeds and the bean to the strawberry mixture. Whisk from time to time until it starts simmering. Lower slightly the heat and let simmer for about 40-60 minutes. Stir occasionally to avoid burning on the bottom.
Test he jam with a drop of juice on a plate: if it doesn’t spill around when the plate is moved, the jam is ready.
Add a pinch of citric acid, stir and remove the pan from the heat.
Fill the jars with the hot jam almost until the top and close them with the lids tightly. Place them on the kitchen counter upside down: this will ensure the lids are perfectly sealed. Let the jars stay this way until they cool until room temperature.
Keep the sealed jars in a dry place at room temperature for up to a year. After opening the jar, keep it in a cool place.