Melt butter in a soup pot over medium heat. Add garlic and sauté until fragrant, about 30 - 60 seconds.
Add flour to pan and whisk to combine. (It will be somewhat lumpy.) Slowly add the milk, a little at a time, whisking constantly to get rid of any lumps. Continue whisking until smooth.
Add chicken broth. Bring soup to a low simmer and cook until thickened, stirring often, about 5 minutes.
Reduce heat to medium-low. Cut potatoes in half length-wise and use a spoon to scoop out potato flesh. Lightly smash the potatoes onto your cutting board or in a bowl, then add to soup. You can continue to smash the potatoes against the side of the pot to get them to your preferred size.
Add 1 ½ cups grated TomaRashi, salt and pepper to soup. Stir until cheese is melted.
Remove pan from heat and stir in ¾ cup sour cream, 2 tablespoons minced chives, and 4-6 strips of the crumbled bacon. (Reserve some of each of these items for serving, if desired). Add additional salt and pepper, to taste.
Ladle into bowls and top with each serving with extra sour cream, onions, cheese, and bacon, if desired.
