Brown onions in a large pot or Dutch oven.

Add diced potatoes, chopped celery, diced carrots, and ham cubes to the pot.

Cover the ingredients with water and add salt to taste.

Bring the mixture to a boil, then reduce heat and simmer for 40 minutes, or until the potatoes are tender.

In a small bowl, whisk together the flour and 1/4 cup of water to create a slurry.

Stir in the evaporated milk to the flour slurry.

Pour the milk and flour mixture into the soup, stirring constantly to prevent lumps.

Cook for an additional 2 minutes, or until the soup has thickened slightly.

Serve hot.