Gas 4. Put the granulated sugar or the caster sugar, whichever you’re using, in an ovenproof bowl and put it in the preheated oven for about 15 minutes to warm through, stirring it once or twice during that time. Place a small plate in the freezer — you will use this later to test the jam to see if it has set.
Meanwhile, put the fresh or frozen strawberries, whichever you’re using, and the lemon juice in a large, wide saucepan set over a medium heat. Using a potato masher, mash the strawberries to the desired consistency — either until they are smooth or, if you prefer, leave some fruit pieces whole. Turn up the heat, bring to the boil and cook for two minutes until juicy.
Add the warmed sugar to the strawberries and lemon juice mixture. Stir until the sugar has dissolved, then boil for another six minutes, stirring frequently. If there is any scum on top (a pale-pink froth, from dust or impurities in the fruit), skim it off with a tablespoon, and discard it, as this could cause the jam to go off.
When the six minutes is up, test to see if your jam is set by putting a teaspoonful of it on the chilled plate you put in the freezer earlier, and setting it aside for one minute. When the jam is cold, run your finger through the ‘blob’ — if a wrinkle forms in the skin on top, then the jam is set. If the jam hasn’t set, boil it for a minute more and test it again on the chilled plate; repeat this process until the jam is set.
Once the jam has set, remove the pan from the heat, and put the jam into sterilised jars — see my Tip on how to sterilise your jars. Cover the jam jars with lids or jam covers while the jam is still hot.