How to cook
Cut the larger strawberries into pieces and put them in a large bowl. Pour sugar over them and stir until evenly covered. Leave for at least 8 hours for them to release their juice.
Pour the strawberries from the bowl with the sugar syrup they've released into a large pot, or in my case into an electric grill pan. Boil on medium heat. Periodically, remove the foam with a spoon.
Mash until you obtain a jam. The boiling time is very relative, there are 2 main ways to determine when it's time to turn the stove off. The 1st is to take a glass of ice cold water and pour a drop of jam into it.
If it doesn't spread out in the water but stays together and sinks, the jam is ready. The other option is to pour a little of the jam into a shallow plate and make a furrow in it. If it doesn't clump back together instantly, it means it's ready.
After removing from the heat, add the citric acid or vinegar. Stir well. Pour into prepared well washed and dried small jars. Close immediately and flip with their caps down.