Heat a large soup pot over medium-low heat. Place the bacon slices flat int he pot and cook, turning a couple times, until crispy and most of the fat has drained from the bacon, 8 to 12 minutes total. Using tongs, transfer the bacon to a paper towel-lined plate, leaving the fat in the pot.

Add the onions, 2 teaspoons salt, and 1 teaspoon pepper. Cook, stirring often, until the onions are translucent and soft, 10 to 12 minutes.

Stir in the potatoes, cauliflower, and stock, cover, and bring to a boil over high heat. Reduce to a strong simmer and cook, stirring here and there, until the potatoes are very soft, 15 to 20 minutes.

Puree the soup, using a blender in batches or a handheld immersion blender. Taste and season with additional salt and pepper, if you like.

Slice or tear the cooked bacon into bite-size pieces and sprinkle it over bowls of the soup. Serve topped with sour cream, chives, and cheese.