Mix the berries with the sugar, lemon juice in a large bowl.
Cover the mixture and let the berries macerate for at least 30 minutes. Stir every so often to help the sugar dissolve. You can also leave them to macerate in the fridge overnight to make jam the next day.
Transfer the macerated fruit and all the juices into a big pot and place the pot with the macerated berries on the stove. Heat on medium, uncovered, stirring every so often.
Once the sugar is completely dissolved, turn off the stove and take the pot off the burner.
Ladle a third of the jam mixture into a blender jar, cover partly, then blend until smooth. Transfer the blended mixture back to the pot.
Bring the mixture to a boil, stirring with a wooden spoon to help distribute the heat.
When the jam comes to a boil, continue cooking it until it hits the jam setting point, around 102-103 °C (215–217 °F).
Take the pot off the stove. Let stand 2 minutes then stir well to distribute the fruit.
Divide the jam among the jars, leaving a ¼ inch headspace. Remove air bubbles, wipe the rims clean, then cover with the lids, fingertight. If any of the jars aren't filled full, don't seal them. Instead you can cover them and enjoy them right away, storing in the fridge.
Seal the closed jars according to the manufacturers recommendations using a water bath canning kit. I used Bernardin jars and I boiled my jam jars for 10 minutes to seal them.
Remove from the canner. Let sit 24 hours undisturbed before storing them in the pantry.
Notes
This recipe makes 4–5 small jars (of 250 mL (1 cup) each)Make sure to sterilize your jars by washing them well and then heating them on a sheet pan in the oven at 250 ºF for at least 30 minutes. Ladle the hot jam into the hot jars.