Prep: Shred the cheese, measure the sour cream, and let them sit at room temperature.
Cook bacon: Add the bacon pieces to a large Dutch oven and heat over medium heat. Cook until crisp, turning as needed (they will overlap, then shrink). Remove the bacon with a slotted spoon. Pour off all but two tablespoons of the grease. Sauté aromatics: Melt the butter in the two tablespoons of bacon grease over medium heat. Add the onions and sauté until soft, about 5 minutes; then add garlic and sauté for 30 seconds. Sprinkle the flour over the top, and cook two additional minutes (it will be thick). Add potatoes: Gradually whisk in the broth while scraping the bottom of the pan. Add the potatoes, evaporated milk, chicken bouillon (without dissolving it in water first), and all the spices. Bring the mixture to a gentle boil and cook, uncovered, for 15-20 minutes, stirring often, until the potatoes are fork-tender.
Make it creamy! Working in batches, remove 1/2 to 3/4 of the soup and transfer it to a blender along with the sour cream. Please don't fill the blender more than 1/3 full to prevent overflow and potential accidents. Blend until very smooth, taking care to allow steam to escape so the blender doesn’t overheat or break. You can do this by leaving the blender cap slightly open and covering it with a paper towel or a loose dish towel. You may also use an immersion blender, but I find the soup doesn’t become as smooth.
Make it cheesy! Stir the pureed soup back into the pot. Stir in milk, more or less, to reach the desired consistency. Stir in the cheddar cheese one handful at a time until melted.
Adjust to taste. Taste and season with more salt or pepper as needed. If you'd like a thinner soup, stir in milk until it reaches the desired consistency. Garnish bowls with cheese, bacon pieces, and chives or green onions.