Place a small plate in the freezer before you start cooking.
To a 3-quart or larger saucepan (the larger the surface area, the shorter the cook time), add the strawberries, rhubarb, sugar, lemon juice and if using, the lemon zest.
1 pound (454 grams) fresh strawberries, 2 cups (250 grams) chopped rhubarb, 1 1/4 cups (250 grams) granulated sugar, 1 tablespoon lemon juice, 1 tablespoon lemon zest
Bring to a boil over medium-high heat while stirring frequently. Turn the heat to the lowest you can to keep it at a full boil.
Let it boil for about 9 minutes, lowering the heat if needed toward the end to prevent the strawberries from burning, until it reaches a temperature of 220°F (104°C). This is the temperature preserves are said to set, but I couldn’t actually get mine above 212°F (100°C). It’ll still be a bit runny but will thicken as it cools and more as it’s chilled.
When you think the preserves are ready, drop a small spoonful onto the cold plate and let it sit for a minute. Push it with your finger - if it wrinkles and holds its shape, the preserves are done.
When it’s done, stir in the vanilla extract.
1/2 teaspoon vanilla extract
Remove the saucepan from the heat and place it on a wire rack to cool completely, about 1–2 hours.
Transfer into clean jars and refrigerate for up to 1 week. Many sources say 2-3 weeks, but I like to play it safe and freeze whatever I haven't eaten after 1 week. It also freezes great and can be frozen for up to 1 year. If freezing, leave some space at the top of the jar for expansion.