Place leek, carrots, celery, broth, salt and pepper in a 6-quart slow cooker. Cover and cook on low for 4 hours until vegetables are tender.
During the final 20 minutes, cook bacon in a skillet over medium heat for 8 to 10 minutes until crisp. Drain and crumble it, keeping it separate from the soup.
Turn the cooker to high and stir in milk. Cover for 15 minutes until the liquid is very hot.
Mix cornstarch with cold water and stir into the cooker with gnocchi. Cover and cook on high for 15 to 25 minutes until gnocchi are tender and the soup lightly thickens, checking early so the dumplings do not fall apart.
Switch off the heat and stir in cheddar in small handfuls. Divide among 6 bowls and top with crisp bacon.