Combine strawberries, rhubarb, and granulated sugar in a Dutch oven. Cook over medium heat for 10-12 minutes until the fruit releases juice and the sugar dissolves, stirring occasionally, then bring the mixture to a gentle simmer with active bubbling.

Stir in lemon juice and powdered pectin until fully combined. Boil for 1-2 minutes at a steady simmer, watching for a thickened, glossy look that coats the back of a spoon.

Check doneness by spooning a small amount onto a cold plate and tilting; it should wrinkle slightly as it cools. Simmer for an additional 3-5 minutes if it remains runny, until the visual cue of a cohesive gel appears.

Remove from heat and stir in vanilla extract if using. Let it stand off the heat for 2-3 minutes so bubbles settle and the texture becomes evenly thick and spreadable.

Ladle the hot jam into clean jars, leaving about 1/4 inch headspace at the top. Wipe rims and seal with lids, then cool at room temperature until set and slightly thickened, showing a spoonable gel.

Refrigerate jam after cooling and use within the recommended time window. For best flavor and set, store chilled and keep lids tightly closed.