Sterilize your jar. Wash your jar and lid in hot soapy water, rinse well, and place upright in a 250°F / 120°C oven for 15 minutes. Remove with tongs and set aside.
Prepare the berries. If using fresh berries, rinse under cold water and pick out any leaves or stems. If using frozen berries, add them directly to the pot — no need to thaw first.
Combine and heat. Place the berries, water, and lemon juice (if using) in a large saucepan. Bring to a boil over medium heat, stirring gently. The berries will begin to soften and release their juice within a few minutes.
Add the sugar. Once the berries are boiling and starting to break down, add the sugar gradually, stirring until fully dissolved. Bring the mixture back to a rolling boil.
Simmer until thickened. Reduce the heat slightly and let the jam simmer for 20–30 minutes, stirring regularly. The berries will turn translucent and the mixture will thicken noticeably. Skim off any foam with a spoon as it forms.
Test for set. Drop a small spoonful of jam onto your cold plate. Wait 30 seconds, then push it with your fingertip. If it wrinkles, the jam is ready. If it slides freely, simmer for another 5 minutes and test again.
Blend (optional). For a smoother jam, use a metal immersion blender directly in the pot. Blend to your preferred consistency — fully smooth or partially chunky both work beautifully. Use caution: the jam is very hot, plastic blender might melt.
Jar the jam. Pour the hot jam carefully into your sterilized jar. Seal immediately. Allow to cool completely at room temperature before refrigerating.
