Sterilise your jars and put 5 teaspoons on a plate in the freezer, to test your jam for doneness later. I sterilise the jars by washing them with washing up liquid and hot water, and then I put them in the oven for 10 minutes at 160 degrees Celsius. I sterilize the jar lids by putting them in boiled water for a couple of minutes and I let them dry on some kitchen towel.
Carefully pick over the blackberries, removing any stray thorns or brambly bits. Rinse them in a bowl. I like to add water and a little bit of salt to it. I then swirl the berries around a bit by hand and then leave it for an hour or so. After that, I pour the water out of the bowl and repeat this process two more times.
Rinse, hull, and roughly chop the strawberries. You can leave the very small strawberries whole.
Combine the berries, sugar, and lemon juice in a large glass or ceramic bowl, cover tightly, and let it rest in the refrigerator for up to 24 hours.
When you’re ready to make the jam, transfer the contents of the bowl to your jam pan. Heat the mixture on medium, stirring frequently until the sugar is fully dissolved. Then turn up the heat and bring the mixture to a boil, stirring as needed to prevent sticking or burning. If it really wants to stick, turn down the heat a bit. In my stainless steel casserole pan, it took me 1,5 hours in total to cook the jam, but the cooking time could take as little as 25 minutes for you. Just make sure to watch the mixture and test it when it starts to thicken up, the foam settles down, and the bubbles become small and shiny.
To test your jam for doneness: Remove the pan from the heat. Use one of your frozen spoons to scoop up a little bit of jam — not a whole spoonful. Return the spoon to the freezer and wait 3 minutes. Retrieve the spoon and hold it vertically. If the mixture just fails to run and is thick and gloppy when you push it with your finger, it’s done. If the jam isn’t ready, cook it a few minutes more.
When the jam is done, bring it back to the point where it just begins to boil and immediately remove it from the heat. Extract some of the seeds from the jam by quickly pressing 3 soup spoons of the very hot mixture through a stainless steel sieve. I use the back of my soup spoon to work the jam through the sieve, holding it directly over the pot.
Ladle or pour the jam into the sterilized jars, leaving about 1 cm at the top. Wipe the jar rims clean and place waxed discs for 1 lb jars on top of the jam. If you want to make sure that the jam really is 'safe' you can also place jam jar covers on top of the jars before securing the lids.