Combine the strawberries, rhubarb, sugar, and lemon juice in a large saucepan. Stir well to evenly distribute the sugar and ingredients.
Place the saucepan over medium heat and bring the mixture to a gentle simmer, stirring frequently. Watch for bubbling appearance and a sweet aroma from the fruit as it starts to soften.
Reduce the heat to low and add the cinnamon. Continue to simmer, stirring occasionally, until the mixture thickens slightly and the fruit is fully softened, about 15-20 minutes. The jam should appear glossy and coat the back of a spoon.
Remove the saucepan from heat and pour the hot jam into sterilized jars or bowls. Let it cool slightly before using or storing.
Once cooled, the jam has a vibrant, chunky appearance with visible fruit pieces suspended in thickened syrup. Serve it spread on bread or as a topping for desserts.