Cut the strawberries in half or quarters leaving any small ones intact and place to one side. With the gooseberries, only cut the large ones in half, try to keep all intact. Keep the fruit separate.
The pan you use for the jam will need to be thick bottomed and high sided. This is because you will be using high heats and the fruit will boil and spit a lot. Molten strawberry hurts! Take the gooseberries and put into the pan. Put in enough water only to prevent burning to the pan, it will be barely a splash. Then cook until tender on a low heat. If you are using fruit that you have frozen earlier in the year then this process will take less time. Fresh fruit will take longer. You are softening them. Don't stir much as it will break up the fruit.
Next add the strawberries to the pan with the gooseberries and add the sugar. Gently mix this and cook on the low heat until all of the sugar is dissolved. At this point all of the juices from the fruits will have released and there will be a surprisingly large amount of liquid. Again, try not to stir much as it will break up the fruits.
Squeeze in the half a lemon, if you are using larger quantities of fruit them just add a little more. Fresh lemon is by far the best and this will not only add to the flavour but will help with the thickening process.
Check the surface of the fruit for bubbles or froth. By adding a tiny amount of butter this will help to smooth the liquid and remove these. You won't need very much and if it isn't enough you can always add a little more. The amount in the picture is a slice rather than a chunk, to give an idea.
Now turn the heat up as high as it will go and boil. If you are using sugar with added pectin you will need to do this for about 10 minutes, possibly a little more. If you are using your own pectin then 2 - 3 minutes. Stir occasionally, or when you can since the jam will be very hot and spitting at this point.
Ignore this step if you have used sugar with added pectin!
remove the pan from the heat and add the pectin. Most bottles of pectin will give you instructions so follow that if you can. If you start with the instructed amount, then drop a small amount of the jam onto a cold plate, wait for a moment to cool and then rub your finger over it. If there is a slight wrinkling to the jam that should be enough. If you are unsure or there isn't, then add more pectin. Repeat this until you are happy.
Tip: if you have used fruit that has been frozen you will need more pectin.
Finally. Leave your jam to cool in the pan for a while. In the meantime sterilize your jars. You can do this many ways. Half fill with water and microwave for a minute, or boil in a pan for a minute on the stove. Whatever you want. Once the jam is cool enough, warm but not hot, pour into your jars and seal.
The butter is optional, if you want this to be dairy free.
If you keep them in a cool dark place, like the back of a cupboard they will last a year or just over. Once opened, keep in the fridge.