Cut off the tops of your strawberries and halve them. Place them in a large bowl, pour in the sugar, cut the lemons into quarters (removing any seeds), squeeze them over the fruit, and then drop the quartered lemons into the bowl.
Stir the whole lot a few times and then cover and leave to macerate overnight.
The next day
Sterilize your jars. (See the note section below for 4 different ways to sterilize.)
Remove the lemons from the bowl of strawberries and discard them. Pour the fruit and the syrup into a jam pan (or large saucepan) and bring to a boil.
Simmer for 10 minutes.
The Wrinkle Test
While the jam is simmering, place one or two small plates in the fridge.
Once the plates are cold, remove them from the fridge and pour a little jam onto one of the cold plates. Wait a few seconds and then push it with your finger. If wrinkles appear on the surface of the jam then your jam is ready to pot. If not, cook for a few minutes longer.
Add a small amount of butter and a pinch of salt and stir gently to dissolve the scum.
Sealing the jars
Ladle into your prepared jars. Leave about a 1/4 of an inch of space from the top of the lid. Stir until the air bubbles are gone from the jam. Secure the lids just until the rings feel snug with a little bit of tension. If you tighten them too much it won't allow for air to escape from the jar which is crucial in the canning process.
Using a deep, large pot, fill it about halfway with water and set it to a low boil.
Place the jars on a rack that fits into your pot and lower into the water making sure that the water covers the jar by about an inch or two. Place upright and don't stack on top of each other. Cover the pot and return to a boil for about 10 minutes or so.
Using tongs or a rack lifter, remove them from the water and place the jars on the countertop covered by a towel for up to 24 hours or until the jars reach room temperature. If there is a slight indentation on the top of the lid, you will know that your jar has a good seal. Refrigerate for up to 10 days.
Notes
4 Ways to Sterilize your jars
Important note: Do not add hot jam into cold jars or cold jam into hot jars. The extreme differing temperatures will cause the glass to shatter.Oven
Line the oven shelves with newspaper (keep away from flame if using gas) and heat the oven to 275 F / 130 C.
Place the jars on top of the newspaper and leave in the oven for at least 20 minutes, then remove using oven mitts.
Dishwash
If your dishwasher gets hot enough, place your clean jars and run a cleaning cycle.
Microwave
DO NOT USE WITH METAL LIDS.
Place your slightly wet (and clean) jars in the microwave for 30 to 45 seconds.
Hot Water Bath
Place your cleaned jars right side up in a pot of water (make sure the water covers them by about an inch) and boil for at least ten minutes.