Heat the olive oil over medium-high heat in a large soup pot, and add the onions and garlic. Cook until the onions are wilted, 4 minutes. Add the potatoes and chicken stock, cover, and bring to a boil. Season with salt and pepper, and add the crushed red pepper. Reduce the heat to a simmer and cook, uncovered, until the potatoes are tender, 20 minutes.
While the potatoes are cooking, thinly slice the kale. Set aside.
When the soup is thick and the potatoes have begun to break down, add the sausage and cook for 5 minutes. Stir in the kale and simmer until the leaves have softened but are still slightly crunchy and the flavors have melded, 15 minutes. Stir in the cilantro, parsley, and mint, and season to taste with salt and pepper. Serve hot.