Start by washing and hulking the strawberries and chopping them into quarter-sized pieces. Trim and slice the rhubarb. Combine both in a large pot.

Sprinkle granulated sugar over the fruit and squeeze in the fresh lemon juice. Stir to combine and let sit for about 10 minutes to macerate.

Cook over medium heat, stirring frequently, until the mixture simmers and thickens, around 30-40 minutes.

Use the chilled plate method to test for doneness. If the jam holds its shape, it's ready; if not, continue simmering.

Pour the hot jam into sterilized jars, seal them, and process in a water bath if storing long-term.