In a large Dutch oven over medium-low heat, melt butter. Stir in diced sweet onion and cook until softened and transparent. Stir in garlic and continue cooking, stirring constantly for 1-2 minutes. 

Whisk in flour and cook for 1- 2 minutes while whisking constantly. Gradually whisk in chicken broth and milk and stir until combined. Allow this mixture to come to a boil, stirring constantly.

Reduce heat to low and add in potatoes, cheddar cheese, and Velveeta. Allow soup to simmer for 15 minutes, stirring often until cheese is completely melted and the soup has thickened.

Stir in bacon, ¾ cup sour cream, salt and pepper. Stir until the sour cream is fully incorporated into the soup. At this point, give the soup a try and add additional salt & pepper to taste.

Serve topped with reserved sour cream, green onion and additional shredded cheese and bacon.