Place a small plate in the freezer.
Prepare ingredients: Wash, hull, and chop the strawberries. Clean the rhubarb stalks and cut them into small pieces. Grate the zest of one lemon and juice both of them. Grate the apple. 1 lb/ 450 g strawberries, 2.2 lbs/ 1 kg rhubarb, 2 lemons, 1 apple Cook: Place strawberries, rhubarb, lemon zest and juice, apple, and sugar in the pan, stir well. Bring to a boil while stirring often, and allow the jam to simmer for about 45-50 minutes, stirring frequently (Note 2), until the mixture thickens and reaches the desired consistency. Adjust the heat if needed to maintain a gentle simmer, where small bubbles break the surface of the liquid without it reaching a rolling boil.7 ½ cups/ 1 ½ kg sugar Skim off foam that will build with a slotted spoon (Note 3).
Start checking the jam after 30 minutes.
Test the consistency by placing a small amount of jam on the plate from the freezer. Run a finger through the middle of the puddle on the plate. If the trail remains and the jam wrinkles, then it’s ready, and you should stop the cooking process. If it doesn’t cook the jam for a few more minutes, check again.
Sterilize jars: While the jam is cooking, sterilize your jam jars by boiling them in water for a few minutes. Sterilize the lids as well. Then, set them aside to dry.
Transfer the jam: Carefully ladle the hot jam into the prepared jars, leaving a small headspace at the top. Wipe any spills or drips from the rims of the jars, then seal the jars.
Process in a water bath or can them in a canner: Place the jars in a large pot of boiling water, ensuring they are fully submerged, and process for about 10 minutes. Remove the jars from the water and let them cool completely. The lids should pop down, indicating a proper seal (Note 4).
Store in a cool, dark place; the jam will last about 1 year. Once opened, refrigerate it and consume it within a few weeks.