Put a small plate into the freezer. This will be used later to test if your jam has set.
Add the washed and chopped fruit, lemon juice, water and sugar to a large high-sided pan set over a medium heat.
Use a wooden spoon to stir the ingredients until the sugar has dissolved then turn the heat to high and bring to a rolling boil. Stir occasionally throughout the cooking process.
Set a timer for 15 minutes. If you have a thermometer, the temperature of the jam should reach 220ºF.
Remove your cold plate from the freezer. Turn the stove off and carefully scoop out a heaped teaspoon of jam and put it on the plate.
Place the plate in fridge for one minute.
Perform the wrinkle test: drag your finger through the jam on the plate. It should noticeably wrinkle. When you remove your finger there should be an indent in the jam and the remaining jam on the plate should not flood the gap.
If the jam is still runny, switch the stove back on and bring the jam back to the boil for two minutes then repeat the wrinkle test.
Give the jam a good stir to disperse any froth.
Let the jam cool for 30 minutes then carefully fill prepared glass jars with jam.
Notes
Fruit: Use fruit that is ripe but not past its prime otherwise it will be mushy.
Lemon juice: It's important to use fresh lemon juice because bottled lemon juice can have an off taste. The lemon juice adds a bit of brightness to the jam, and it also helps the jam to set.
Sugar: A good rule of thumb is to use the same amount of sugar to prepared fruit ratio. I weighed the rhubarb and strawberries and and they came to approximately 500 grams of fruit. This is very close to 2 and 1/2 cups of granulated sugar. If you want to be very precise, then use this method.
Serving size: The serving size for this recipe is approximately one ounce of jam.