Place several small saucers in the freezer at least 1 hour before you start. Always begin by sterilizing your jars and lids and use them straight from the oven.
Wash, peel, core (where needed) and cut your fruit into bite-sized pieces, about 1cm ( ½ in) cubes, leaving berries whole (if not too large). Heat and soften the fruit first, until it becomes pulp. For whole chunky bits in your jam, keep some fruit pieces aside for later in the process. Add the recipe’s water measurement and the lemon juice to help it soften. If using frozen fruit, defrost it first and don’t add the water, as it retains a lot in the freezing process. Only stir now and again at this stage, so it doesn’t stick to the bottom of the pan. Different fruits are stickier, so be vigilant and never walk away from your jam.
When it is pulpy and bubbling, add the sugar, stirring until it has completely dissolved. Add any of the extra whole or chunks of fruit here, so they don’t break down as much. Turn up the heat to the highest setting, and only stir intermittently, not allowing too much cold air into the mix. There’s a fine line between stirring to ensure it’s not catching and burning and stirring too much and lowering its temperature. Ideally, you want the jam to get to a temperature of 104 C (220 F). I find thermometers unreliable, so to judge when my jam is ready for set/wrinkle testing and jarring, I boil it on the highest heat until the rolling bubbles thicken, becoming slower and ploppier. This is the heat struggling to break through to the surface as the density of the jam becomes thicker and “jams up”. Timings always differ, depending on the fruit pectin levels, the pan you are using, and the heat source – even the temperature of the room can affect your timings – so use your eyes and instinct to make a judgement.
Add the elderflower cordial at the end, after skimming off any foam, so that the flavour doesn’t burn out while cooking.
Jams can be stored in a cool, dark place for up to 6–12 months. Once opened, store in the fridge and eat within 4–6 weeks.