Add peeled and cubed potatoes to a large pot. Fill with water until it's one inch above the potatoes. Add 1 teaspoons of salt to the water. Cover and place over medium-high heat until water is boiling. Remove cover and continue cooking until al dente. Drain and set aside until later in the recipe.
Diced bacon into small pieces. Cook in a Dutch oven over medium heat until crispy, about 8 minutes. Scoop out cooked bacon and drain on paper towels. Do not discard bacon drippings.
Add butter and onion bacon drippings and continue to cook over medium heat until softened, about 8 minutes. Do not let the onions burn so make sure you stir occasionally. Add in garlic and cook until fragrant, about 30 seconds.
Add flour to onion mixture and whisk together. Continue to whisk for about 2 minutes. Slowly add milk to the mixture to roux to let it thicken up. Do the same with the chicken broth. Bring soup back up to a simmer. Add in the potatoes and stir.
Add cheese to the soup in 1/4-cup bunches, whisking after every addition. This prevents clumping and helps the cheese melt evenly. Add half of the cooked bacon and the sour cream. Stir to combined. Taste the soup and then season, as needed.
Serve soup immediately with the rest of the cooked bacon, sliced green onions, and more cheese on top.