Coat the cooking insert of a 5- to 6-qt. slow-cooker* all over with cooking-oil spray. Evenly spread caramel on bottom, then top with an even layer of pears. Sprinkle with cranberries.
Whisk together flour, sugar, salt, baking powder, milk, and butter in a medium bowl until mostly smooth with only tiny lumps remaining. Pour over fruit and spread with a spatula to cover evenly.
Cover and cook until a toothpick inserted in cake portion comes out clean, about 1 hour and 20 minutes on high or 2 1/4 hours on low.
Remove basin from slow-cooker and let pudding cake cool 5 minutes. Spoon onto plates, pear side up, and dollop with crème fraîche.
*You can use a 4-qt. slow-cooker, but may need to increase the cooking time.