Warm a large heavy-bottomed pot over medium-high heat and drizzle a little oil. Cook the spicy Italian sausage, breaking it up until browned and cooked through. Remove the sausage and drain the excess grease.
In the same pot, fry the diced bacon in the leftover sausage fat until crisp, then transfer the bacon to a plate with the sausage. Leave about 1 to 1 1/2 tablespoons of bacon grease in the pot.
Add the chopped onion to the pot and sauté for roughly 5 minutes until softened, stirring in minced garlic during the last minute.
Sprinkle flour over the cooked onion and garlic, stirring to coat everything evenly. Slowly pour in the chicken stock while stirring.
Add the diced potatoes to the pot and increase the heat to bring the soup to a boil. Let it simmer gently for about 15 minutes or until the potatoes are easily pierced with a fork.
Stir the cooked sausage back into the soup along with most of the bacon, heavy cream, and torn kale. Cook for another 5 to 10 minutes until the kale has softened.
Season the soup with salt, pepper, and if desired, red pepper flakes to taste. Garnish with the remaining crispy bacon and serve warm.