Set a large Dutch oven over medium-high heat. Add the bacon and cook until crispy, 6 to 8 minutes; remove to a plate. When cool, crumble and set aside for garnish.
Add the onions and garlic to the pot, and cook in the rendered bacon grease until softened, 4 to 5 minutes. Add the flour and some salt and pepper, and stir until combined. Cook for 3 minutes, so the raw flour taste cooks out. Add the chicken broth and heavy cream, whisking to remove any flour lumps. Add the potatoes and cook over medium heat until tender, about 10 minutes.
With a potato masher, break up the cooked potatoes¿this will yield a thick and chunky consistency. Whisk in sour cream and then the shredded cheddar; let it melt through.
Serve garnished with crispy crumbled bacon and sliced scallions.