Begin by finely chopping half of a large yellow onion and dicing your leftover ham into bite-sized cubes. Next, slice a few carrots and stalks of celery for added texture. Lastly, peel and cube russet or Yukon gold potatoes into ¾-inch pieces.

In a heavy pot, melt 2 tablespoons of butter over medium heat. Add the chopped onion, sautéing until it turns translucent, about 5-7 minutes. Then, toss in the minced garlic and stir for another minute until fragrant.

Fold in the sliced carrots and celery into the sautéed onion mixture, cooking for an additional minute. Once the vegetables are slightly softened, sprinkle in ¼ cup of flour, stirring well to combine.

Carefully transfer the sautéed mixture from the pot into your slow cooker. Then, pour in 3 cups of milk, whisking to integrate the flour fully and avoid any lumps.

Add 4 cups of chicken or vegetable stock to the slow cooker, along with 2 bay leaves, the cubed potatoes, and diced ham. Stir the mixture well to ensure that the ingredients are evenly distributed.

Cover your slow cooker and set it to cook on low for 8 hours or high for 4 hours.

Once the cooking time is complete, check the soup to ensure the potatoes are tender. Remove the bay leaves before serving, and give the soup a good stir.