(recipe slightly adapted from Gourmet Quick Kitchen Magazine )
Heat 1 tablespoon olive oil in a 5 quart heavy pot over medium heat and brown sausage, stirring constantly for 3 minutes.
Transfer sausage with a slotted spoon to a plate. Add 2 tablespoons of olive oil to the pot (with the reserved fat) and cook the onion & garlic with a heavy pinch of salt & pepper as well as a pinch of red pepper flakes over medium heat. Cook until browned, about 7 to 8 minutes.
Add potatoes, water and 1 teaspoon of salt and simmer, covered until potatoes are tender, 15 to 20 minutes.
Mash some of the potatoes into the soup to thicken, then add the kale and simmer uncovered, until tender, about 5 minutes. Stir in sausage and cool until just heated through, 1 to 2 minutes. Finish with 1 teaspoon of red wine vinegar and season with salt & pepper to taste. Drizzle with remaining olive oil.