Place two metal spoons into the freezer. These will be used later to check if the jam is set.
First, prepare the fruit. Wash and hull the strawberries and chop them into small pieces. (If using frozen strawberries, there’s no need to defrost or chop. The frozen fruit will break down as it heats up.) Peel the kiwi and chop into small pieces. Place all of the fruit in a large saucepan.
Add ¼ cup of water to the fruit and bring to a simmer. Cook for 5-10 minutes until the strawberries and kiwis have softened and released their juices. If you prefer preserves with more texture, continue to the next step. Otherwise, use an immersion blender to partially puree the fruit mixture to the desired smoothness. (Or carefully transfer the fruit to a blender and puree, then put it back in the pot.)
Mix 3 tablespoons of pectin with about ¼ cup of the measured sugar (to prevent clumping). Add the pectin/sugar mixture and lemon juice to the pureed fruit and bring to a boil. Boil for 1 minute, stirring occasionally.
Combine the rest of the sugar with the fruit and pectin. Bring to a full boil and allow to cook for 1 minute to ensure the sugar is dissolved, stirring occasionally so the fruit doesn’t burn.
Check for jell. Remove one of the spoons from the freezer. Scoop up a little jam and give it a few seconds to cool. If the warm jam on the spoon looks viscous and you can see little wrinkles of surface tension, then it’s set. If the jam is still too runny, add 1 more tablespoon of pectin and boil for 1 minute. Check for jell again using the remaining spoon. Repeat as necessary to get the desired consistency.
After checking if the jam is set, also give it a taste. Now is the time to adjust for sweetness. If you want the jam to be sweeter, add more sugar ¼ cup at a time, always boiling for a minute to make sure the sugar dissolves. Keep in mind that the longer you cook the jam, the more it will lose its fresh, fruity flavor.