Hull the strawberries and crush them using a potato masher in a medium saucepan. Add the lemon juice and bring this to a simmer over medium-high heat and cook for about five minutes, stirring occasionally. Add the sugar and return to a simmer over medium heat, stirring often. If the mixture begins to foam, add the butter, let the foam subside, but skim off any that remains. Stir in the pectin, simmer for one minute, then pour into a clean jar or container, cool and then refrigerate.
Alternatively the jam can be jarred for storage in your pantry, following safe canning procedures.
The refrigerated jam will keep for up to six months.