Generously spray your 6-quart slow cooker insert with nonstick cooking spray or line it with a slow cooker liner for easy cleanup.
Layer half of the cubed bread in the bottom of the slow cooker. Evenly spread half of the diced fresh strawberries over the bread layer. Repeat with the remaining bread and strawberries.
In a large bowl, whisk together the strawberry preserves, milk, eggs, vanilla extract, and brown sugar until well combined and the sugar is dissolved.
Pour the egg mixture evenly over the layered bread and strawberries in the slow cooker, ensuring all bread cubes are coated. Gently press the bread down to help it absorb the liquid.
Cover the slow cooker and refrigerate for at least 4 hours, or ideally, overnight. This allows the bread to fully soak up the custardy mixture.
When ready to cook, place the covered slow cooker on your counter. Cook on LOW for 4-6 hours or on HIGH for 2-3 hours, until the casserole is golden brown around the edges and set in the center. A knife inserted should come out mostly clean.