This soup is probably my mom's favorite thing that I've ever made. The funny thing is, it's inspired by her guilty pleasure: the Olive Garden. I can see why she likes it, though—it's hearty, with bright green kale, homemade sausage, and a little Parmesan to take the savory flavor to a whole new dimension.
For the sausage: Heat a dry skillet and toast the fennel seeds until fragrant. Add toasted fennel to the pork along with the Espelette pepper, red chili flakes and a big pinch of salt. Toss and set aside. Over high heat, coat the bottom of a soup pot or Dutch oven with olive oil. Position the meat grinder over the pot. Add 5 whole garlic cloves and the onions to the meat mixture. Turn the grinder on high and run the sausage mixture through the grinder. Use the plunger if needed to push the meat through and into the heated soup pot to brown.
For the soup: When the sausage has begun to brown and caramelize, add the onions and garlic, roughly chopped, to the pot. Stir the sausage mixture so it doesn't stick to the bottom; then add olive oil and cook 2-3 more minutes. Add the stock and a pinch of salt, scrape the bottom of the pot with a wooden spoon, and cover. Bring to a rolling boil over high heat.
As the soup comes to a boil, thinly slice the potatoes on a mandoline. Stir them into the soup, making sure the slices come apart. Add plenty of freshly ground black pepper and another pinch of salt. Cover, bring back to a rolling boil, and continue to let boil for 20 minutes.
Finish the soup: Reduce heat to low and remove lid. The potatoes should be broken down and the soup thickened. Adjust the seasoning with salt and pepper; then stir in the vinegar. Pull the fibrous stem out of the kale leaves and discard; roughly chop the leaves and add to the soup to wilt. Ladle into bowls and serve with more pepper, salt, a drizzle of olive oil, and a fresh grating of Parmesan.