Homemade Cheesy Potato Soup

Melt butter in a Dutch oven over medium-high heat. Add onion; cook and stir until tender, 5 minutes. Add potatoes and water; bring to a boil. Reduce heat; cover and simmer until potatoes are tender, 15 minutes. Stir in the milk, soup, garlic salt and pepper; heat until warmed through. Add cheese; stir until cheese is melted. Sprinkle with parsley.