Heat the olive oil in a large Dutch oven or soup pot over medium heat.
Add the turkey sausage. Cook for 5–7 minutes, breaking it into small pieces with a wooden spoon, until browned and cooked through.
Add the diced onion. Cook for 4–5 minutes, stirring occasionally, until the vegetables begin to soften.
Stir in the garlic, Italian seasoning, crushed red pepper flakes, salt, and black pepper. Cook for about 30 seconds, or until fragrant.
Pour in the chicken broth and diced tomatoes. Stir in the cannellini beans and cauliflower.
Bring the soup to a gentle boil. Reduce the heat to medium-low, cover partially, and simmer for 15–20 minutes, or until the cauliflower is tender.
Stir in the chopped kale and simmer for another 3–5 minutes, or until the kale is tender and bright green.
Remove the soup from the heat. Stir in the lemon juice and Parmesan cheese.
Taste and adjust the seasonings as needed. Ladle the soup into bowls and top with additional Parmesan and fresh parsley, if desired.