Place the raw strawberry halves in an even layer on the bottom of a 4- to 6-quart slow cooker. The berries should mostly cover the bottom; a few gaps are fine.
Halved strawberries layered in a slow cooker
Halved strawberries layered in a slow cooker
In a small bowl, stir together the granulated sugar and cornstarch until no white clumps of cornstarch remain. This keeps the thickener from clumping when it hits the juicy berries.
Sprinkle the sugar–cornstarch mixture evenly over the strawberries in the slow cooker. Drizzle the vanilla extract over the top.
Using a large spoon, gently toss the strawberries right in the slow cooker until they are evenly coated with the sugar mixture and vanilla. Spread them back into an even layer on the bottom.
Strawberries being gently tossed with sugar in the slow cooker
Strawberries being gently tossed with sugar in the slow cooker
Cover the slow cooker with the lid and cook on LOW for 2 to 3 hours, until the strawberries are very soft, swimming in their own syrup, and the liquid has thickened slightly. Avoid lifting the lid during the first 1 1/2 hours so the mixture can come up to temperature properly.
Once the berries are tender and glossy, give them a gentle stir. If the syrup seems a bit thin, cook uncovered on HIGH for 15 to 20 minutes, stirring once or twice, until slightly thickened to your liking.
