Brown the spicy sausage over medium heat in a large heavy pot, stirring/flipping occasionally until cooked through. Use a slotted spoon to transfer the sausage to a plate lined with paper towels to absorb some of the fat. Drain any of the remaining sausage fat from the skillet.
Heat the olive oil in the same pot of medium heat. Add the onion and cook, stirring occasionally, for 8-10 minutes. Add the garlic, salt, and black pepper, continuing cooking another minute, then add the celery, fennel, and cauliflower florets. Let the vegetables continue to sweat, stirring occasionally, another 6-7 minutes. Stir a little bit of the chicken stock into the vegetables, scraping up any browned bits from the bottom of the pan. Add the rest of the chicken stock, bring to a bowl, then reduce the heat to a simmer and cook for 15 minutes.
Let the soup cool briefly, add the paprika, then carefully transfer the soup to a high-speed blender in batches and blend until fully smooth.
To serve from a single pot, add the soup back to the original pot and stir in the spinach and cooked spicy sausage. To serve individual bowls, place a handful of spinach leaves at the bottom of each bowl, ladle two scoops of soup on top, and decorate with a scattering of the pork sausage.